How is the mass fraction of a substance calculated? Mass fraction of an element in a complex substance - Knowledge Hypermarket

Solution A homogeneous mixture of two or more components is called.

Substances that are mixed to form a solution are called components.

The components of the solution are solute, which may be more than one, and solvent. For example, in the case of a solution of sugar in water, sugar is the solute and water is the solvent.

Sometimes the concept of solvent can be applied equally to any of the components. For example, this applies to those solutions that are obtained by mixing two or more liquids that are ideally soluble in each other. So, in particular, in a solution consisting of alcohol and water, both alcohol and water can be called a solvent. However, most often in relation to water-containing solutions, it is traditionally customary to call water a solvent, and the second component the solute.

As a quantitative characteristic of the composition of the solution, such a concept is most often used as mass fraction substances in solution. The mass fraction of a substance is the ratio of the mass of this substance to the mass of the solution in which it is contained:

where ω (in-va) - mass fraction of the substance contained in the solution (g), m(v-va) - the mass of the substance contained in the solution (g), m (p-ra) - the mass of the solution (g).

From formula (1) it follows that the mass fraction can take values from 0 to 1, that is, it is a fraction of a unit. Concerning mass fraction can also be expressed as a percentage (%), and it is in this format that it appears in almost all tasks. The mass fraction, expressed as a percentage, is calculated using a formula similar to formula (1), with the only difference that the ratio of the mass of the solute to the mass of the entire solution is multiplied by 100%:

For a solution consisting of only two components, the mass fractions of the solute ω(r.v.) and the mass fraction of the solvent ω(solvent) can be respectively calculated.

The mass fraction of a solute is also called solution concentration.

For a two-component solution, its mass is the sum of the masses of the solute and the solvent:

Also in the case of a two-component solution, the sum of the mass fractions of the solute and solvent is always 100%:

Obviously, in addition to the formulas written above, one should also know all those formulas that are directly mathematically derived from them. For example:

It is also necessary to remember the formula that relates the mass, volume and density of a substance:

m = ρ∙V

and you also need to know that the density of water is 1 g / ml. For this reason, the volume of water in milliliters is numerically equal to the mass of water in grams. For example, 10 ml of water has a mass of 10 g, 200 ml - 200 g, etc.

In order to successfully solve problems, in addition to knowing the above formulas, it is extremely important to bring the skills of their application to automaticity. This can only be achieved by solving a large number varied tasks. Tasks from real USE exams on the topic "Calculations using the concept of" mass fraction of a substance in solution "" can be solved.

Examples of tasks for solutions

Example 1

Calculate the mass fraction of potassium nitrate in a solution obtained by mixing 5 g of salt and 20 g of water.

Decision:

The solute in our case is potassium nitrate, and the solvent is water. Therefore, formulas (2) and (3) can be written respectively as:

From the condition m (KNO 3) \u003d 5 g, and m (H 2 O) \u003d 20 g, therefore:

Example 2

What mass of water must be added to 20 g of glucose to obtain a 10% glucose solution.

Decision:

It follows from the conditions of the problem that the solute is glucose, and the solvent is water. Then formula (4) can be written in our case as follows:

From the condition, we know the mass fraction (concentration) of glucose and the mass of glucose itself. Denoting the mass of water as x g, we can write the following equivalent equation based on the formula above:

Solving this equation we find x:

those. m(H 2 O) \u003d x g \u003d 180 g

Answer: m (H 2 O) \u003d 180 g

Example 3

150 g of a 15% sodium chloride solution were mixed with 100 g of a 20% solution of the same salt. What is the mass fraction of salt in the resulting solution? Give your answer to the nearest integer.

Decision:

To solve problems for the preparation of solutions, it is convenient to use the following table:

	1st solution	2nd solution	3rd solution
m r.v.
m solution
ω r.v.

where m r.v. , m r-ra and ω r.v. are the values of the mass of the dissolved substance, the mass of the solution and the mass fraction of the dissolved substance, respectively, individual for each of the solutions.

From the condition, we know that:

m (1) solution = 150 g,

ω (1) r.v. = 15%,

m (2) solution = 100 g,

ω (1) r.v. = 20%,

Inserting all these values into the table, we get:

We should remember the following formulas necessary for calculations:

ω r.v. = 100% ∙ m r.v. /m solution, m r.v. = m r-ra ∙ ω r.v. / 100% , m solution = 100% ∙ m r.v. /ω r.v.

Let's start filling out the table.

If only one value is missing in a row or column, then it can be counted. The exception is the line with ω r.v., knowing the values in two of its cells, the value in the third one cannot be calculated.

The first column is missing a value in only one cell. So we can calculate it:

m (1) r.v. = m (1) r-ra ∙ ω (1) r.v. /100% = 150 g ∙ 15%/100% = 22.5 g

Similarly, we know the values in two cells of the second column, which means:

m (2) r.v. = m (2) r-ra ∙ ω (2) r.v. /100% = 100 g ∙ 20%/100% = 20 g

Let's enter the calculated values in the table:

Now we have two values in the first line and two values in the second line. So we can calculate the missing values (m (3) r.v. and m (3) r-ra):

m (3) r.v. = m (1) r.v. + m (2)r.v. = 22.5 g + 20 g = 42.5 g

m (3) solution = m (1) solution + m (2) solution = 150 g + 100 g = 250 g.

Let's enter the calculated values in the table, we get:

Now we have come close to calculating the desired value ω (3) r.v. . In the column where it is located, the contents of the other two cells are known, so we can calculate it:

ω (3)r.v. = 100% ∙ m (3) r.v. / m (3) solution = 100% ∙ 42.5 g / 250 g = 17%

Example 4

To 200 g of a 15% sodium chloride solution was added 50 ml of water. What is the mass fraction of salt in the resulting solution. Give your answer to the nearest hundredth _______%

Decision:

First of all, you should pay attention to the fact that instead of the mass of added water, we are given its volume. We calculate its mass, knowing that the density of water is 1 g / ml:

m ext. (H 2 O) = V ext. (H 2 O) ∙ ρ (H2O) = 50 ml ∙ 1 g/ml = 50 g

If we consider water as a 0% sodium chloride solution containing, respectively, 0 g of sodium chloride, the problem can be solved using the same table as in the example above. Let's draw such a table and insert the values we know into it:

In the first column, two values are known, so we can calculate the third:

m (1) r.v. = m (1)r-ra ∙ ω (1)r.v. /100% = 200 g ∙ 15%/100% = 30 g,

In the second line, two values \u200b\u200bare also known, so we can calculate the third:

m (3) solution = m (1) solution + m (2) solution = 200 g + 50 g = 250 g,

Enter the calculated values in the appropriate cells:

Now two values in the first line have become known, which means we can calculate the value of m (3) r.v. in the third cell:

m (3) r.v. = m (1) r.v. + m (2)r.v. = 30 g + 0 g = 30 g

ω (3)r.v. = 30/250 ∙ 100% = 12%.

Mass fraction of an element in matter- This is one of the topics that is included in the course of chemistry. Skills and abilities to determine this parameter can be useful when testing knowledge during control and independent work, as well as the exam in chemistry.

You will need

- periodic system of chemical elements D.I. Mendeleev

Instruction

In order to calculate the mass share, you must first find the relative atomic mass (Ar) of the desired element, as well as the relative molecular mass (Mr) of the substance. Next, apply the formula by which the mass fraction of the element is determined (W) W \u003d Ar (x) / Mr x 100%, in which W is the mass fraction of the element (measured in fractions or%); Ar (x) is the relative atomic mass of the element; Mr is the relative molecular mass of a substance. To determine the relative atomic and molecular mass, use the periodic system of chemical elements of D.I. Mendeleev. When calculating, be sure to take into account the number of atoms of each element.
Example #1: Determine the Mass share hydrogen in water. Find according to the table D.I. Mendeleev relative atomic mass of hydrogen Ar (H) = 1. Since there are 2 hydrogen atoms in the formula, therefore, 2Ar (H) = 1 x 2 = 2 Calculate the relative molecular mass of water (H2O), which is made up of 2 Ar (H) and 1 Ar (O). Mr (H2O) \u003d 2Ar (H) + Ar (O)Ar (O) \u003d 16, therefore Mr (H2O) \u003d 1 x 2 + 16 \u003d 18
Write down the general formula for determining the mass fraction of an element W \u003d Ar (x) / Mr x 100% Now write down the formula, in relation to the condition of the problem W (H) \u003d 2 Ar (H) / Mr (H2O) x 100% Make calculations W (H) \u003d 2 / 18 x 100% = 11.1%
Example #2: Determine the Mass share oxygen in copper sulfate (CuSO4). Find according to the table D.I. Mendeleev, the relative atomic mass of oxygen Ar (O) \u003d 16. Since there are 4 oxygen atoms in the formula, therefore, 4 Ar (O) \u003d 4 x 16 \u003d 64 Calculate the relative molecular weight of copper sulfate (CuSO4), which consists of 1 Ar (Cu), 1 Ar (S) and 4 Ar (O).Mr (CuSO4) = Ar (Cu) + Ar (S) + 4 Ar (O).Ar (Cu) = 64 Ar (S) = 324 Ar (O) = 4 x 16 \u003d 64, therefore Mr (CuSO4) \u003d 64 + 32 + 64 \u003d 160
Write down the general formula for determining the mass fraction of an element W \u003d Ar (x) / Mr x 100% Now write down the formula, in relation to the condition of the problem W (O) \u003d 4 Ar (O) / Mr (CuSO4) x 100% Make calculations W (O) \u003d 64 / 160 x 100% = 40%

Chemistry is definitely an interesting science. Despite all its complexity, it allows us to better understand the nature of the world around us. And what's more - at least elementary knowledge in this subject seriously helps in everyday life. For example, the determination of the mass fraction of a substance in a multicomponent system, that is, the ratio of the mass of any component to the total mass of the entire mixture.

Necessary:

- calculator;
- scales (if you first need to determine the masses of all components of the mixture);
is Mendeleev's periodic system of elements.

Instructions:

So, it became necessary for you to determine the mass fraction of a substance. Where to begin? First of all, it depends on the specific task and on the tools at hand for the job. But in any case, in order to determine the content of a component in a mixture, you need to know its mass and the total mass of the mixture. You can do this either on the basis of known data, or on the basis of your own research. To do this, you will need to weigh the added component on a laboratory scale. After the mixture is ready, weigh it also.
mass the right substance write as " m«, total mass systems put under the designation " M". In this case, the formula for the mass fraction of a substance will take the following form: W=(m/M)*100. The result obtained is recorded as a percentage.
Example: calculate the mass fraction of 15 grams of salt dissolved in 115 grams of water. Solution: the total mass of the solution is determined by the formula M=m to +m c, where m in- mass of water mc- mass of salt. From simple calculations, it can be determined that the total mass of the solution is 130 grams. According to the above definition formula, we obtain that the content of table salt in the solution will be equal to W=(15/130)*100=12%.
A more particular situation is the need to define mass fraction of a chemical element in a substance . It is defined exactly the same. Main principle the calculation will remain the same, only instead of the mass of the mixture and the specific component, you will have to deal with the molecular weights of the chemical elements.
All the necessary information can be found in the periodic system of Mendeleev. Break down the chemical formula of a substance into its main components. Using the periodic table, determine the mass of each element. Summing them up, get the molecular weight of your substance ( M). Similarly to the previous case, the mass fraction of a substance, or, to be more precise, an element, will be determined by the ratio of its mass to the molecular mass. The formula will take the following form W=(m a /M)*100. Where m a is the atomic mass of the element, M is the molecular weight of the substance.
Let's consider this case with a specific example. Example: determine the mass fraction of potassium in potash. Potash is potassium carbonate. Its formula K2CO3. The atomic mass of potassium is 39 , carbon - 12 , oxygen - 16 . The molecular weight of carbonate will be determined as follows - M \u003d 2m K + m C + 2m O \u003d 2 * 39 + 12 + 2 * 16 \u003d 122. A potassium carbonate molecule contains two potassium atoms with an atomic mass equal to 39 . The mass fraction of potassium in the substance will be determined by the formula W \u003d (2m K / M) * 100 \u003d (2 * 39 / 122) * 100 \u003d 63.93%.

Instruction

Determine the chemical form of the substance, the mass fractions of the elements of which you want to find. Take the periodic system of Mendeleev and find in it the cells of the elements corresponding to the atoms that make up the molecule of this substance. In the cell, find the mass number of each such element. If the found value of the mass number element fractional, round it up to the nearest .

In the case when atoms of the same type occur several times in a molecule, multiply their atomic mass by this number. Add the masses of all the elements that make up the molecule to get the value in atomic mass units. For example, if you need to find the mass of a salt molecule, which is sulfate (Na2SO4), determines the atomic mass of sodium Ar(Na)=23, sulfur Ar(S)=32 and Ar(O)=16. Since the molecule contains 2 sodium, then for it take the value 23 * 2 = 46, and, which has 4 atoms - 16 * 4 = 64. Then the mass of the molecule will be sodium sulfate and will be Мr(Na2SO4)=46+32+64=142.

To calculate the mass fractions of the elements that make up the molecule of a given substance, find the ratio of the masses of the atoms that make up the molecule of the substance to the mass of the molecule, and multiply the result by 100%. For example, if we consider sodium sulfate Na2SO4, calculate the mass fractions of its elements as follows: - the mass fraction of sodium will be ω(Na)= 23 2 100%/142=32.4%;
- mass fraction of sulfur will be ω(S)= 32 100%/142=22.5%;
- the mass fraction of oxygen will be ω(О)= 16 4 100%/142=45.1%.

Mass fractions show the relative elements in a given molecule of a substance. Check the correctness of the calculation by adding the mass fractions of the substance. Their sum should be 100%. In the example under consideration, 32.4% + 22.5% + 45.1% \u003d 100%, the calculation was made.

Perhaps it is impossible to find an element as necessary for life as oxygen. If a person can live without food for several weeks, without water for several days, then without oxygen - only a few minutes. This substance is found wide application in various industries, including chemical, as well as a component of rocket fuel (oxidizer).

Instruction

Often there is a need to determine the mass of oxygen in a closed volume, or as a result chemical reaction. For example: 20 grams of permanganate was subjected to thermal decomposition, the reaction went to the end. How many grams of oxygen were released?

First of all, remember that potassium - aka - has the chemical formula KMnO4. When heated, it decomposes, forming potassium manganate - K2MnO4, the main one - MnO2, and O2. Having written the reaction equation, and choosing the coefficients, you get:

2KMnO4 = K2MnO4 + MnO2 + O2

Given that the approximate molecular weight of two potassium permanganate molecules is 316, and the molecular weight of an oxygen molecule, respectively, is 32, by solving the proportion, calculate:

20 * 32 /316 = 2,02
That is, with the thermal decomposition of 20 grams of potassium permanganate, approximately 2.02 grams of oxygen is obtained. (Or rounded 2 grams).

Or, for example, it is required to determine the mass of oxygen in a closed volume, if its temperature and pressure are known. This is where the universal Mendeleev-Clapeyron equation comes to the rescue, or in other words, the “ideal gas equation of state”. It looks like this:

PVm = MRT
P is the gas pressure,

V is its volume,

m is its molar mass,

M - mass,

R is the universal gas constant,

T is temperature.

You see that the required value, that is, the mass of gas (oxygen), after bringing all the initial data into one system of units (pressure - , temperature - in degrees Kelvin, etc.), can be easily calculated by the formula:

Of course, real oxygen is not an ideal gas, for the description of which this equation was introduced. But at pressure and temperature values close to , the deviations of the calculated values from the actual ones are so insignificant that they can be safely neglected.

Instruction

1. The mass fraction of a substance is found by the formula: w \u003d m (c) / m (cm), where w is the mass fraction of the substance, m (c) is the mass of the substance, m (cm) is the mass of the mixture. If the substance is dissolved, then the formula looks like this: w \u003d m (c) / m (p-ra), where m (p-ra) is the mass of the solution. The mass of the solution, if necessary, can also be detected: m (p-ra) \u003d m (c) + m (p-la), where m (p-la) is the mass of the solvent. If desired, the mass fraction can be multiplied by 100%.

2. If the value of the mass is not given in the condition of the problem, then it can be calculated with the support of several formulas, the data in the condition will help to choose the appropriate one. The first formula for finding mass is: m = V*p, where m is mass, V is volume, p is density. The further formula looks like this: m = n * M, where m is the mass, n is the number of substance, M is the molar mass. The molar mass, in turn, is made up of the nuclear masses of the elements that make up the substance.

3. For a better understanding of this material, let's solve the problem. A mixture of copper and magnesium filings weighing 1.5 g was treated with an excess of sulfuric acid. As a result of the reaction, hydrogen was released in a volume of 0.56 l (typical data). Calculate the mass fraction of copper in the mixture. In this problem, a reaction takes place, we write down its equation. Of the 2 substances, only magnesium interacts with an excess of hydrochloric acid: Mg + 2HCl = MgCl2 + H2. In order to find the mass fraction of copper in the mixture, you need to substitute the values \u200b\u200binto the following formula: w (Cu) \u003d m (Cu) / m (cm). The mass of the mixture is given, we find the mass of copper: m (Cu) \u003d m (cm) - m (Mg). We are looking for the mass of magnesium: m (Mg) \u003d n (Mg) * M (Mg). The reaction equation will help to find the number of magnesium substance. We find the number of hydrogen substance: n \u003d V / Vm \u003d 0.56 / 22.4 \u003d 0.025 mol. The equation shows that n(H2) = n(Mg) = 0.025 mol. We calculate the mass of magnesium, knowing that the molar mass of magnesium is 24 g / mol: m (Mg) \u003d 0.025 * 24 \u003d 0.6 g. We find the mass of copper: m (Cu) \u003d 1.5 - 0.6 \u003d 0.9 g It remains to calculate the mass fraction: w (Cu) \u003d 0.9 / 1.5 \u003d 0.6 or 60%.

Mass fraction shows as a percentage or in fractions the content of the substance in any solution or element in the composition of the substance. Knowing how to calculate the mass fraction is beneficial not only in chemistry lessons, but also when you want to prepare a solution or mixture, say, for culinary purposes. Or change the percentage, in the composition that you already have.

Instruction

1. The mass fraction is calculated as the ratio of the mass of a given component to the total mass of the solution. To acquire the total as a percentage, you need to multiply the resulting quotient by 100. The formula looks like this:? = m (solute) / m (solution)?,% =? * 100

2. Let's consider for example the direct and inverse problems. Let's say you dissolved 5 grams of table salt in 100 grams of water. What percentage solution did you get? The solution is very primitive. You know the mass of the substance (salt), the mass of the solution will be equal to the sum of the masses of water and salt. Thus, you should divide 5 g by 105 g and multiply the result of the division by 100 - this will be the result: you will get a 4.7% solution. Now the inverse problem. You want to prepare 200 grams of a 10% aqueous solution of what is desired. How much substance to take to dissolve? We act in the reverse order, we divide the mass fraction expressed as a percentage (10%) by 100. We get 0.1. Now let's make a simple equation, where we denote the required number of substances x and, consequently, the mass of the solution as 200 g + x. Our equation will look like this: 0.1=x/200g+x. When we solve it, we get that x is approximately 22.2 g. The result is checked by solving the direct problem.

3. It is more difficult to find out what numbers of solutions of a known percentage must be taken to acquire a certain number of solutions with new given qualities. Here it is required to compose and solve a system of equations. In this system, the first equation is an expression of the famous mass of the resulting mixture, in terms of two unfamiliar masses of the initial solutions. Let's say, if our goal is to get 150 g of a solution, the equation will look like x + y \u003d 150 g. The second equation is the mass of the solute equal to the sum of the same substance, as part of 2 miscible solutions. Let's say if you want to have a 30% solution, and the solutions that you mix are 100%, that is, a pure substance, and 15%, then the second equation will look like: x + 0.15y \u003d 45 g. for a little, solve the equation system and find out how much substance needs to be added to a 15% solution in order to get a 30% solution. Try it.

Related videos

To calculate amount substances, find out its mass with the help of scales, express it in grams and divide by the molar mass, which can be detected with the support of the periodic table. To determine the number substances gas under typical conditions, apply Avogadro's law. If the gas is in other conditions, measure the pressure, volume and temperature of the gas, then calculate amount substances in him.

You will need

You will need scales, a thermometer, a manometer, a ruler or tape measure, the periodic table of Mendeleev.

Instruction

1. Definition of a number substances in a solid or liquid. Find the mass of the investigated body with the help of scales, express it in grams. Determine from which substances the body is composed, then with the support of the periodic table, detect the molar mass substances. To do this, find the elements that make up the molecule substances of which the body is made. According to the table, determine their nuclear masses, if the table indicates a fractional number, round it up to a whole number. Find the sum of the masses of all the atoms in the molecule substances, get the molecular weight, which is numerically equal to the molar mass substances in grams per mole. After this, divide the previously measured mass by the molar mass. As a result, you will get amount substances in moles (?=m/M).

2. Number substances gas under typical conditions. If the gas is in typical conditions (0 degrees Celsius and 760 mmHg), detect its volume. To do this, measure the volume of the room, cylinder or vessel where it is located, from the fact that the gas occupies each volume provided to it. In order to get its value, measure the geometric dimensions of the vessel, where it is located with the support of a tape measure and with the support of mathematical formulas, find its volume. A particularly classic case is the parallelepiped-shaped room. Measure its length, width and height in meters, then multiply them and get the volume of gas that is in it. cubic meters. To discover amount substances gas, divide the resulting volume by the number 0.0224 - the molar volume of gas under typical conditions.

3. Number substances gas with arbitrary parameters. Measure the pressure of the gas with a pressure gauge in pascals, its temperature in kelvins, for which add the number 273 to the degrees Celsius in which the thermometer measures. Also determine the volume of gas in cubic meters. To discover amount substances divide the product of pressure and volume by temperature and the number 8.31 (universal gas continuous), ? = PV / (RT).

Related videos

Many liquids are solutions. These are, in particular, human blood, tea, coffee, sea water. The basis of the solution is the solute. There are tasks to find the mass fraction of this substance.

Instruction

1. Solutions are called homogeneous homogeneous systems, which consist of 2 or more components. They are divided into three categories: - liquid solutions; - solid solutions; - gaseous solutions. Liquid solutions include, say, dilute sulfuric acid, solid solutions include an alloy of iron and copper, and gaseous solutions include any mixture of gases. Regardless of the state of aggregation of the solution, it consists of a solvent and a solute. The most common solvent is usually water, with which the substance is diluted. The composition of solutions is expressed in different ways, especially often the value of the mass fraction of the solute is used for this. The mass fraction is a dimensionless quantity, and it is equal to the ratio of the mass of the solute to the total mass of each solution: To calculate given parameter as a percentage, use the following formula: w (substance) \u003d m in / m (solution) 100%. To find the same parameter in the form decimal fraction do not multiply by 100%.

2. The mass of each solution is the sum of the masses of water and solute. Consequently, occasionally the above formula is written in a slightly different way: solute is an acid. It follows from this that the mass of the solute is calculated as follows:? in \u003d mHNO3 / mHNO3 + mH2O

3. If the mass of the substance is unknown, and only the mass of water is given, then in this case the mass fraction is found according to a slightly different formula. When the volume of the solute is known, find its mass using the following formula: mv \u003d V *? From this it follows that the mass fraction of the substance is calculated as follows:? v \u003d V *? / V *?

4. Finding the mass fraction of a substance is repeatedly carried out for utilitarian purposes. Say, when bleaching some material, you need to know the concentration of perhydrol in a peroxide solution. In addition, the exact calculation of the mass fraction is occasionally required in medical practice. In addition to formulas and an approximate calculation of the mass fraction in medicine, they also use experimental verification with the help of instruments, which can reduce the likelihood of errors.

5. There are several physical processes during which the mass fraction of a substance and the composition of the solution change. The first of these, called evaporation, is a process inverse to the dissolution of a substance in water. In this case, the solute remains, and the water is completely evaporated. In this case, the mass fraction cannot be measured - there is no solution. The exact opposite process is the dilution of a concentrated solution. The more it is diluted, the more strongly the mass fraction of the substance dissolved in it decreases. Concentration is a partial evaporation, in which not all water evaporates, but only part of it. The mass fraction of the substance in the solution increases in this case.

Related videos

What is mass fraction element? From the name itself, it is possible to realize that this is a value indicating the ratio of the mass element, which is part of the substance, and the total mass of this substance. It is expressed in fractions of a unit: percent (hundredths), ppm (thousandths), etc. How is it possible to calculate the mass of any element ?

Instruction

1. For clarity, take a look at carbon, well-known to everyone, without which there would be no organics. If carbon is a pure substance (say, diamond), then its mass share it is allowed to bravely take it as a unit or for 100%. Of course, diamond also contains impurities of other elements, but in most cases, in such small numbers that they can be neglected. But in such modifications of carbon as coal or graphite, the content of impurities is quite high, and such ignoring is unacceptable.

2. If carbon is part of a difficult substance, you need to do it in the following way: write down the exact formula of the substance, after that, knowing the molar masses of any element included in its composition, calculate the exact molar mass of this substance (of course, taking into account the "index" of any element). Later this determine the mass share by dividing the total molar mass element on the molar mass of the substance.

3. Let's say we need to find a mass share carbon in acetic acid. Write the formula for acetic acid: CH3COOH. To simplify the calculations, convert it to the form: С2Н4О2. The molar mass of this substance is the sum of the molar masses of the elements: 24 + 4 + 32 = 60. Accordingly, the mass fraction of carbon in this substance is calculated as follows: 24/60 = 0.4.

4. If you need to calculate it as a percentage, respectively, 0.4 * 100 = 40%. That is, every kilogram of acetic acid contains (approximately) 400 grams of carbon.

5. Of course, in exactly the same way it is possible to detect mass fractions of all other elements. Say, the mass fraction of oxygen in the same acetic acid is calculated as follows: 32/60 \u003d 0.533, or approximately 53.3%; and the mass fraction of hydrogen is 4/60 = 0.666 or approximately 6.7%.

6. To check the accuracy of the calculations, add up the percentages of all elements: 40% (carbon) + 53.3% (oxygen) + 6.7% (hydrogen) = 100%. The account settled.

You have a two hundred liter barrel. You plan to completely fill it with diesel fuel, which you use to heat your mini-boiler room. And how much will it weigh, filled with solarium? Now let's calculate.

You will need

- table of specific density of substances;
– knowledge to make the simplest mathematical calculations.

Instruction

1. In order to find the mass of a substance by its volume, use the formula for the specific density of a substance. p \u003d m / v here p is the specific density of the substance; m is its mass; v is the volume occupied. We will consider the mass in grams, kilograms and tons. Volumes in cubic centimeters, decimeters and measures. And specific gravity, respectively, in g/cm3, kg/dm3, kg/m3, t/m3.

2. It turns out, according to the conditions of the problem, you have a two-hundred-liter barrel. This means: a barrel with a capacity of 2 m3. It is called a two-hundred-liter one, since water, with its specific gravity equal to one, enters 200 liters into such a barrel. You are concerned about the mass. Therefore, bring it to the first place in the presented formula. m \u003d p * v On the right side of the formula, the value of p is unknown - specific gravity diesel fuel. Find it in the directory. Even easier is to search the Internet with a query “specific gravity of diesel fuel”.

3. Found: the density of summer diesel fuel at t = +200 C - 860 kg / m3. Substitute the values in the formula: m = 860 * 2 = 1720 (kg) 1 ton and 720 kg - 200 liters of summer diesel fuel weigh so much. Having hung the barrel in advance, it is allowed to calculate the total weight and estimate the capacity of the rack under the barrel with a solarium.

4. In rural areas, it can be useful to pre-calculate the mass of firewood needed by cubic capacity in order to determine the carrying capacity of the transport on which this firewood will be delivered. For example, you need at least 15 cubic meters for the winter. meters of birch firewood. Look in the reference literature for the density of birch firewood. This is: 650 kg / m3. Calculate the mass by substituting the values \u200b\u200binto the same specific density formula. m \u003d 650 * 15 \u003d 9750 (kg) Now, based on the carrying capacity and body capacity, you can decide on the type of vehicle and the number of trips.

Related videos

Note!
Older people are more familiar with the representation of specific gravity. Specific gravity substances is the same as specific gravity.

The mass fraction of a substance shows its table of contents in a more difficult structure, say, in an alloy or mixture. If the total mass of a mixture or alloy is known, then knowing the mass fractions of the constituent substances, it is possible to detect their masses. To detect the mass fraction of a substance, it is possible to know its mass and the mass of each mixture. This value can be expressed in fractional units or percentages.

You will need

scales;
periodic table of chemical elements;
calculator.

Instruction

1. Determine the mass fraction of the substance that is in the mixture through the masses of the mixture and the substance itself. To do this, with the support of weights, determine the masses of substances that make up a mixture or alloy. Then fold them up. Take the resulting mass as 100%. To find the mass fraction of a substance in a mixture, divide its mass m by the mass of the mixture M, and multiply the result by 100% (?%=(m/M)?100%). Let's say that 20 g of table salt are dissolved in 140 g of water. In order to find the mass fraction of salt, add the masses of these 2 substances M=140+20=160 g. After that, find the mass fraction of the substance?%=(20/160)?100%=12.5%.

2. If you need to find the table of contents or the mass fraction of an element in a substance with a known formula, use the periodic table of chemical elements. Use it to find the nuclear masses of the elements that make up the substance. If one element occurs several times in the formula, multiply its nuclear mass by this number and add up the totals. This will be the molecular weight of the substance. In order to find the mass fraction of any element in such a substance, divide its mass number in the given chemical formula M0 by the molecular weight of the given substance M. Multiply the result by 100% (?%=(M0/M)?100%).

3. Say, determine the mass fraction of chemical elements in copper sulphate. Copper sulfate (copper II sulfate) has the chemical formula CuSO4. The nuclear masses of the elements included in its composition are equal to Ar(Cu)=64, Ar(S)=32, Ar(O)=16, the mass numbers of these elements will be equal to M0(Cu)=64, M0(S)=32, M0(O)=16?4=64, taking into account that the molecule contains 4 oxygen atoms. Calculate the molecular weight of a substance, it is equal to the sum of the mass numbers of the substances that make up the molecule 64+32+64=160. Determine the mass fraction of copper (Cu) in the composition of copper sulfate (?%=(64/160)?100%)=40%. According to the same thesis, it is possible to determine the mass fractions of all elements in this substance. Mass fraction of sulfur (S) ?%=(32/160)?100%=20%, oxygen (O) ?%=(64/160)?100%=40%. Please note that the sum of all mass fractions of the substance must be 100%.

Mass fraction is the percentage content of a component in a mixture or an element in a substance. Not only schoolchildren and students face the tasks of calculating the mass fraction. The knowledge to calculate the percentage concentration of a substance finds absolutely utilitarian use and in real life- where the preparation of solutions is required - from construction to cooking.

You will need

- periodic table;
- formulas for calculating the mass fraction.

Instruction

1. Calculate the mass share a-priory. Because the mass of a substance is made up of the masses of the elements that make it up, then share of any constituent element, a certain part of the mass of the substance is brought. The mass fraction of a solution is equal to the ratio of the mass of the solute to the mass of each solution.

2. The mass of the solution is equal to the sum of the masses of the solvent (traditionally water) and the substance. The mass fraction of the mixture is equal to the ratio of the mass of the substance to the mass of the mixture containing the substance. Multiply the result by 100%.

3. detect mass share output with the support of the formula?=md/mp, where mp and md are the value of the supposed and actual obtained yield of the substance (mass), respectively. Calculate the assumed mass from the reaction equation using the formula m=nM, where n is the chemical number of the substance, M is the molar mass of the substance (the sum of the nuclear masses of all elements included in the substance), or the formula m=V?, where V is the volume of the substance, ? is its density. In turn, if necessary, replace the number of substances with the formula n \u003d V / Vm, or also find it from the reaction equation.

4. Mass share calculate the element of a difficult substance using the periodic table. Add up the nuclear masses of all the elements that make up the substance, multiplying by the indices if necessary. You will get the molar mass of the substance. Find the molar mass of an element from the periodic table. Calculate the mass share by dividing the molar mass of the element by the molar mass of the substance. Multiply by 100%.

Helpful advice
Pay attention to the physical process, the one that takes place. When evaporating, do not calculate the mass fraction, because there is no solution (water or any other liquid). Do not forget that during concentration, on the contrary, called partial evaporation, the mass fraction of the substance increases. If you dilute a concentrated solution, the mass fraction decreases.

The mass fraction of any component in a substance shows which part of the total mass is brought to the atoms of this particular element. Using the chemical formula of a substance and the periodic table of Mendeleev, it is possible to determine the mass fraction of all of the elements included in the formula. The resulting value is expressed as an ordinary fraction or percentage.

Instruction

1. If you want to determine the mass fraction of any element that makes up a chemical formula, start by calculating the number of atoms that is brought to all of the elements. Let's say the chemical formula of ethanol is written like this: CH?-CH?-OH. And the chemical formula of dimethyl ether is CH?-O-CH?. The number of oxygen atoms (O) in any of the formulas is one, carbon (C) - two, hydrogen (H) - six. Note that these are different substances because the identical number of atoms of the entire element in their molecules are arranged differently. However, the mass fractions of the entire element in dimethyl ether and ethanol will be identical.

2. Using the periodic table, determine the nuclear mass of each element included in the chemical formula. Multiply this number by the number of atoms of each element calculated in the previous step. In the example used above, the formula contains one oxygen atom each, and its atomic mass from the table is 15.9994. There are two carbon atoms in the formula, its atomic mass is 12.0108, which means that the total weight of the atoms will be 12.0108*2=24.0216. For hydrogen these numbers are 6, 1.00795 and 1.00795*6=6.0477, respectively.

3. Determine the total atomic mass of the entire molecule of the substance - add the numbers obtained in the previous step. For dimethyl ether and ethanol, this value should be equal to 15.9994+24.0216+6.0477=46.0687.

4. If you want to get the total in fractions of a unit, make up an individual fraction for each element included in the formula. Its numerator should contain the value calculated for this element in the second step, and put the number from the third step in the denominator of the whole fraction. The resulting ordinary fraction can be rounded to the required degree of accuracy. In the example used above, the mass fraction of oxygen is 15.9994/46.0687?16/46=8/23, carbon is 24.0216/46.0687?24/46=12/23, hydrogen is 6.0477/46, 0687?6/46=3/23.

5. To get the total as a percentage, convert the resulting ordinary fractions to decimal format and increase by a hundred times. In the example used, the mass fraction of oxygen in percent is expressed by the number 8/23 * 100? 34.8%, carbon - 12/23 * 100? 52.2%, hydrogen - 3/23 * 100? 13.0%.

Related videos

Note!
The mass fraction cannot be greater than one or, if it is expressed as a percentage, greater than 100%.