Knowing the chemical formula, you can calculate the mass fraction of chemical elements in a substance. element in substance is denoted in Greek. letter “omega” - ω E/V and is calculated using the formula:

where k is the number of atoms of this element in the molecule.

What is the mass fraction of hydrogen and oxygen in water (H 2 O)?

Solution:

M r (H 2 O) = 2*A r (H) + 1*A r (O) = 2*1 + 1* 16 = 18

2) Calculate the mass fraction of hydrogen in water:

3) Calculate the mass fraction of oxygen in water. Since water contains atoms of only two chemical elements, the mass fraction of oxygen will be equal to:

Rice. 1. Formulation of the solution to problem 1

Calculate the mass fraction of elements in the substance H 3 PO 4.

1) Calculate the relative molecular mass of the substance:

M r (H 3 PO 4) = 3*A r (N) + 1*A r (P) + 4*A r (O) = 3*1 + 1* 31 +4*16 = 98

2) Calculate the mass fraction of hydrogen in the substance:

3) Calculate the mass fraction of phosphorus in the substance:

4) Calculate the mass fraction of oxygen in the substance:

1. Collection of problems and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry, 8th grade” / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M.: AST: Astrel, 2006.

2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006. (p. 34-36)

3. Chemistry: 8th grade: textbook. for general education institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§15)

4. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed.V.A. Volodin, Ved. scientific ed. I. Leenson. - M.: Avanta+, 2003.

1. Unified collection of digital educational resources ().

2. Electronic version of the journal “Chemistry and Life” ().

4. Video lesson on the topic “Mass fraction of a chemical element in a substance” ().

Homework

1. p.78 No. 2 from the textbook “Chemistry: 8th grade” (P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005).

2. With. 34-36 No. 3.5 from the Workbook in Chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. 8th grade” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M.: AST: Astrel: Profizdat, 2006.

Chemistry is definitely an interesting science. Despite all its complexity, it allows us to better understand the nature of the world around us. And moreover, at least basic knowledge of this subject seriously helps in everyday life. For example, determining 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);
— Mendeleev’s periodic table of elements.

Instructions:

• So, it became necessary for you to determine the mass fraction of the substance. Where to start? First of all, it depends on the specific task and on the tools at hand for the work. But in any case, to determine the content of a component in a mixture, you need to know its mass and the total mass of the mixture. This can be done 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 prepared, weigh it as well.
• Write down the mass of the required substance as “ m«, total mass put the systems under the designation “ M". In this case, the formula for the mass fraction of the 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 table salt dissolved in 115 g 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, m c- mass of table salt. From simple calculations it can be determined that the total mass of the solution is 130 grams. Using the above determination formula, we find 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 determine mass fraction of a chemical element in a substance . It is defined in exactly the same way. The main principle of 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 masses of the chemical elements.
• All the necessary information can be found in the periodic table of Mendeleev. Break down the chemical formula of a substance into its main components. Using the periodic table, determine the mass of each element. By summing them up, you get the molecular mass of your substance ( M). Similar 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 its molecular mass. The formula will take the following form W=(m a /M)*100. Where m a- atomic mass of the element, M- molecular weight of the substance.
• Let's look at this case using a specific example. Example: determine the mass fraction of potassium in potash. Potash is potassium carbonate. Its formula K2CO3. Atomic mass of potassium - 39 , carbon - 12 , oxygen - 16 . The molecular weight of the carbonate will be determined as follows - M = 2m K +m C +2m O = 2*39+12+2*16 = 122. The 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 = (2m K /M)*100 = (2*39/122)*100 = 63.93%.

The mass fraction of an element ω(E)% is the ratio of the mass of a given element m (E) in a given molecule of a substance to the molecular mass of this substance Mr (in-va).

The mass fraction of an element is expressed in fractions of a unit or as a percentage:

ω(E) = m (E) / Mr(in-va) (1)

ω% (E) = m(E) 100%/Mr(in-va)

The sum of the mass fractions of all elements of a substance is equal to 1 or 100%.

As a rule, to calculate the mass fraction of an element, they take a portion of a substance equal to the molar mass of the substance, then the mass of a given element in this portion is equal to its molar mass multiplied by the number of atoms of a given element in the molecule.

So, for a substance A x B y in fractions of unity:

ω(A) = Ar(E) X / Мr(in-va) (2)

From proportion (2) we derive a calculation formula for determining the indices (x, y) in the chemical formula of a substance if the mass fractions of both elements and the molar mass of the substance are known:

X = ω%(A) Mr(in-va) / Ar(E) 100% (3)

Dividing ω% (A) by ω% (B), i.e. transforming formula (2), we obtain:

ω(A) / ω(B) = X Ar(A) / Y Ar(B) (4)

Calculation formula (4) can be transformed as follows:

X: Y = ω%(A) / Ar(A) : ω%(B) / Ar(B) = X(A) : Y(B) (5)

Calculation formulas (3) and (5) are used to determine the formula of a substance.

If the number of atoms in a molecule of a substance for one of the elements and its mass fraction are known, the molar mass of the substance can be determined:

Mr(v-va) = Ar(E) X / W(A)

Examples of solving problems on calculating the mass fractions of chemical elements in a complex substance

Calculation of mass fractions of chemical elements in a complex substance

Example 1. Determine the mass fractions of chemical elements in sulfuric acid H 2 SO 4 and express them as percentages.

Solution

1. Calculate the relative molecular weight of sulfuric acid:

Mr (H 2 SO 4) = 1 2 + 32 + 16 4 = 98

2. Calculate the mass fractions of elements.

To do this, the numerical value of the mass of the element (taking into account the index) is divided by the molar mass of the substance:

Taking this into account and denoting the mass fraction of an element with the letter ω, calculations of mass fractions are carried out as follows:

ω(H) = 2: 98 = 0.0204, or 2.04%;

ω(S) = 32: 98 = 0.3265, or 32.65%;

ω(O) = 64: 98 =0.6531, or 65.31%

Example 2. Determine the mass fractions of chemical elements in aluminum oxide Al 2 O 3 and express them as percentages.

Solution

1. Calculate the relative molecular weight of aluminum oxide:

Mr(Al 2 O 3) = 27 2 + 16 3 = 102

2. Calculate the mass fractions of elements:

ω(Al) = 54: 102 = 0.53 = 53%

ω(O) = 48: 102 = 0.47 = 47%

How to calculate the mass fraction of a substance in a crystalline hydrate

Mass fraction of a substance is the ratio of the mass of a given substance in a system to the mass of the entire system, i.e. ω(X) = m(X) / m,

where ω(X) is the mass fraction of substance X,

m(X) - mass of substance X,

m - mass of the entire system

Mass fraction is a dimensionless quantity. It is expressed as a fraction of a unit or as a percentage.

Example 1. Determine the mass fraction of water of crystallization in barium chloride dihydrate BaCl 2 2H 2 O.

Solution

The molar mass of BaCl 2 2H 2 O is:

M(BaCl 2 2H 2 O) = 137+ 2 35.5 + 2 18 = 244 g/mol

From the formula BaCl 2 2H 2 O it follows that 1 mol of barium chloride dihydrate contains 2 mol H 2 O. From this we can determine the mass of water contained in BaCl 2 2H 2 O:

m(H2O) = 2 18 = 36 g.

We find the mass fraction of water of crystallization in barium chloride dihydrate BaCl 2 2H 2 O.

ω(H 2 O) = m(H 2 O)/m(BaCl 2 2H 2 O) = 36 / 244 = 0.1475 = 14.75%.

Example 2. Silver weighing 5.4 g was isolated from a rock sample weighing 25 g containing the mineral argentite Ag 2 S. Determine the mass fraction of argentite in the sample.

Even one gram of a substance can contain up to a thousand different compounds. Each compound is responsible for a specific property of a substance, and it happens that it is not a specific substance, but a mixture. In any case, in production there is often a situation of recycling chemical waste and the task of using secondary raw materials. It is the chemical reactions that make it possible to find and isolate a certain substance that are dominant. But to do this, you must first learn how to find the mass fraction.

The concept of mass fraction of a substance reflects its content and concentration in a complex chemical structure, be it a mixture or an alloy. Knowing the total mass of an alloy or mixture, you can find the masses of their constituent substances, provided that their mass fractions are known. How to find the mass fraction, the formula is usually expressed as a fraction: mass fraction of a substance mass of a substance / mass of the entire mixture.

Let's do a little experiment! To do this, we will need a periodic table of chemical elements. Mendeleev, scales and calculator.

How to find the mass fraction of a substance

It is necessary to determine the mass fraction of the substance; the substance is in the form of a mixture. First, we put the substance itself on the scale. We obtained a mass of substance. Knowing a certain mass of a substance in a mixture, we can easily obtain its mass fraction. For example, there is 170g. water. They contain 30 grams of cherry juice. Total weight=170+30=230 grams. Let's divide the mass of cherry juice to the total mass of the mixture: 30/200=0.15 or 15%.

How to find the mass fraction of a solution

A solution to this problem may be needed when determining the concentration of food solutions (vinegar) or medications. The mass of a KOH solution, also known as potassium hydroxide, weighing 400 grams is given. KOH (mass of the substance itself) is 80 grams. It is necessary to find the mass fraction of bile in the resulting solution. Formula for finding the solution: KOH (mass of potassium hydroxide solution) 300 g, mass of dissolved substance (KOH) 40 g. Find KOH (mass fraction of alkali) in the resulting solution, t-mass fraction. m- mass, t (substance) = 100%* m (substance) / m (solution (substance). Thus KOH (mass fraction of potassium hydroxide solution): t (KOH) = 80 g / 400 g x 100% = 20 %.

How to find the mass fraction of carbon in a hydrocarbon

To do this, we use the periodic table. We look for substances in the table. The table shows the atomic mass of the elements. 6 carbons with atomic mass 12 and 12 hydrogens with atomic mass equal to 1. m (C6H12) = 6 x 12 + 12 x 1 = 84 g/mol, ω (C) = 6 m1(C) / m (C6H12) = 6 x 12 / 84 = 85%

Determination of the mass fraction in production is carried out in special chemical laboratories. To begin with, a small sample is taken and various chemical reactions are tested. Or they introduce litmus tests that can show the presence of one or another component. After determining the initial structure of the substance, the isolation of components can begin. This is achieved through simple chemical reactions, when one substance comes into contact with another and a new one is obtained, a precipitate is possible. There are also more advanced methods, such as electrolysis, heating, cooling, evaporation. Such reactions require large industrial equipment. Production, of course, can hardly be called environmentally friendly, however, modern waste treatment technologies make it possible to minimize the burden on nature.

Mass fraction of the element in substance- This is one of the topics that is included in the chemistry course. The skills and abilities to determine this parameter can be useful when testing knowledge during tests and independent work, as well as on the Unified State Exam in chemistry.

You will need

• - periodic table of chemical elements D.I. Mendeleev

Instructions

• 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 that determines the mass fraction of the element (W)W = 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 table of chemical elements D.I. Mendeleev. When doing your calculations, be sure to take into account the number of atoms of each element.
• Example No. 1. Determine the mass share hydrogen in water. Find from the table D.I. Mendeleev relative atomic mass of hydrogen Ar (H) = 1. Since in the formula there are 2 hydrogen atoms, therefore, 2Ar (H) = 1 x 2 = 2 Calculate the relative molecular mass of water (H2O), which is the sum of 2 Ar (H) and 1 Ar (O).Mr (H2O) = 2Ar (H) + Ar (O)Ar (O) = 16, thereforeMr (H2O) = 1 x 2 + 16 = 18
• Write down the general formula for determining the mass fraction of an element W = Ar (x) / Mr x 100% Now write the formula in relation to the problem condition W (H) = 2 Ar (H) / Mr (H2O) x 100% Make calculations W (H) = 2 / 18 x 100% = 11.1%
• Example No. 2. Determine the mass share oxygen in copper sulfate (CuSO4). Find from the table D.I. Mendeleev's relative atomic mass of oxygen Ar (O) = 16. Since there are 4 oxygen atoms in the formula, therefore, 4 Ar (O) = 4 x 16 = 64 Calculate the relative molecular mass of copper sulfate (CuSO4), which is the sum 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 = 64, therefore Mr (CuSO4) = 64 + 32 + 64 = 160
• Write down the general formula for determining the mass fraction of an element W = Ar (x) / Mr x 100% Now write the formula in relation to the problem condition W (O) = 4 Ar (O) / Mr (CuSO4) x 100% Make calculations W (O) = 64 / 160 x 100% = 40%