How to calculate the mass fraction of an element. Mass fraction of an element in a complex substance

Fractions of solute
ω = m1 / m,
where m1 is the mass of the solute and m is the mass of the entire solution.

If the mass fraction of the solute is needed, multiply the resulting number by 100%:
ω \u003d m1 / m x 100%

In tasks where you need to calculate the mass fractions of each of the elements included in the chemical, use the table D.I. Mendeleev. For example, find out the mass fractions of each of the elements that make up the hydrocarbon, which C6H12

m (C6H12) \u003d 6 x 12 + 12 x 1 \u003d 84 g / mol
ω (C) \u003d 6 m1 (C) / m (C6H12) x 100% \u003d 6 x 12 g / 84 g / mol x 100% \u003d 85%
ω (H) \u003d 12 m1 (H) / m (C6H12) x 100% \u003d 12 x 1 g / 84 g / mol x 100% \u003d 15%

Helpful advice

Solve the problems of finding the mass fraction of a substance after evaporation, dilution, concentration, mixing of solutions using formulas obtained from the determination of the mass fraction. For example, the problem of evaporation can be solved using the following formula
ω 2 \u003d m1 / (m - Dm) \u003d (ω 1 m) / (m - Dm), where ω 2 is the mass fraction of the substance in the evaporated solution, Dm is the difference between the masses before and after heating.

Sources:

• how to determine the mass fraction of a substance

There are situations when it is necessary to calculate mass liquids contained in any container. This can be during a training session in the laboratory, and in the course of solving a household problem, for example, when repairing or painting.

Instruction

The easiest method is to resort to weighing. First, weigh the container together with, then pour the liquid into another container that is suitable in size and weigh the empty container. And then it remains only to subtract the smaller value from the larger value, and you get. Of course, this method can be resorted to only when dealing with non-viscous liquids, which, after overflow, practically do not remain on the walls and bottom of the first container. That is, the quantity will remain then, but it will be so small that it can be neglected, it will hardly affect the accuracy of the calculations.

And if the liquid is viscous, for example,? How then her mass? In this case, you need to know its density (ρ) and occupied volume (V). And then everything is elementary. Mass (M) is calculated from M = ρV. Of course, before calculating it is necessary to convert the factors into a single system of units.

Density liquids can be found in a physical or chemical reference book. But it is better to use a measuring device - a density meter (densitometer). And the volume can be calculated, knowing the shape and overall dimensions of the container (if it has the correct geometric shape). For example, if the same glycerin is in a cylindrical barrel with a base diameter d and a height h, then the volume

1. Fill in the gaps in the sentences.

a) In mathematics, "share" is the ratio of a part to a whole. To calculate the mass fraction of an element, multiply its relative atomic mass by the number of atoms of a given element in the formula and divide by the relative molecular mass of the substance.

b) The sum of the mass fractions of all elements that make up the substance is 1 or 100%.

2. Write down mathematical formulas for finding the mass fractions of elements if:

a) the formula of the substance is P 2 O 5, M r \u003d 2 * 31 + 5 * 16 \u003d 142
w(P) = 2*31/132 *100% = 44%
w(O) = 5*16/142*100% = 56% or w(O) = 100-44=56.

b) the formula of the substance - A x B y
w(A) = Ar(A)*x/Mr(AxBy) * 100%
w(B) = Ar(B)*y / Mr(AxBy) *100%

3. Calculate mass fractions of elements:

a) in methane (CH 4)

b) in sodium carbonate (Na 2 CO 3)

4. Compare the mass fractions of the indicated elements in substances and put a sign<, >or = :

5. In the combination of silicon with hydrogen, the mass fraction of silicon is 87.5%, hydrogen 12.5%. The relative molecular weight of the substance is 32. Determine the formula of this compound.

6. Mass fractions of elements in the compound are shown in the diagram:

Determine the formula of this substance if it is known that its relative molecular weight is 100.

7. Ethylene is a natural fruit ripening stimulant: its accumulation in fruits accelerates their ripening. The earlier the accumulation of ethylene begins, the earlier the fruits ripen. Therefore, ethylene is used to artificially accelerate fruit ripening. Derive the formula of ethylene if it is known that the mass fraction of carbon is 85.7%, the mass fraction of hydrogen is -14.3%. The relative molecular weight of this substance is 28.

8. Derive the chemical formula of the substance, if it is known that

a) w(Ca) = 36%, w(Cl) = 64%

b) w(Na) 29.1%, w(S) = 40.5%, w(O) = 30.4%.

9. Lapis has antimicrobial properties. Previously, it was used to cauterize warts. In small concentrations, it acts as an anti-inflammatory and astringent, but can cause burns. Derive the formula of lapis if it is known that it contains 63.53% silver, 8.24% nitrogen, 28.23% oxygen.

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

The concept of the mass fraction of a substance reflects its content and concentration in a complex chemical structure, whether it be a mixture or an alloy. Knowing the total mass of an alloy or mixture, one 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: the mass fraction of a substance is the mass of the substance / the mass of the entire mixture.

Let's do a little experiment! To do this, we need the 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. Initially, we put the substance itself on the scales. Got a lot of stuff. 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. Divide the mass of cherry juice by the total mass of the mixture: 30/200=0.15 or 15%.

How to find the mass fraction of a solution

The solution to this problem may be needed when determining the concentration of food solutions (vinegar) or drugs. Given the mass of a solution of KOH, also known as potassium hydroxide, weighing 400 grams. KOH (the mass of the substance itself) is 80 grams. It is necessary to find the mass fraction of bile in the resulting solution. Solution formula: KOH (mass of potassium hydroxide solution) 300 g, mass of solute (KOH) 40 g. Find KOH (mass fraction of alkali) in the resulting solution, t is 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 are looking for substances in the table. The table shows the atomic mass of the elements. 6 carbons with an atomic mass of 12 and 12 hydrogens with an atomic mass of 1. m (C6H12) \u003d 6 x 12 + 12 x 1 \u003d 84 g / mol, ω (C) \u003d 6 m1 (C) / m (C6H12) \u003d 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, on which various chemical reactions are tested. Or they introduce litmus tests, which can show the presence of a particular component. After clarifying the initial structure of the substance, you can begin to isolate the components. This is achieved through simple chemical reactions, when one substance comes into contact with another and a new one is obtained, precipitation is possible. There are 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.

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Mass fraction of an element in a complex substance

The paragraph will help you:

> find out what the mass fraction of an element in a compound is and determine its value;
> calculate the mass of the element in a certain mass of the compound, based on the mass fraction of the element;
> correctly formulate the solution of chemical problems.

Every difficult substance(chemical compound) is formed by several elements. To know the content of the elements in the compound is necessary for its effective use. For example, the best nitrogen fertilizer is considered to be the one that contains the largest amount of Nitrogen (this element is necessary for plants). Similarly, the quality of metal ore is assessed, determining how much it " rich» on a metal element.

Content element in the compound characterize its mass fraction. This value is denoted by the Latin letter w (“double-ve”).

Let us derive a formula for calculating the mass fraction of an element in a compound from the known masses of the compound and the element. We denote the mass fraction of the element by the letter x. Taking into account that the mass of the compound is a whole, and the mass of an element is a part of the whole, we make up the proportion:

Note that the masses of the element and the compound must be taken in the same units of measurement (for example, in grams).

It is interesting

In two Sulfur compounds - SO 2 and MoS 3 - the mass fractions of the elements are the same and amount to 0.5 (or 50%) each.

The mass fraction has no dimension. It is often expressed as a percentage. In this case formula takes this form:

It is obvious that the sum of the mass fractions of all elements in the compound is 1 (or 100%).

Let us give several examples of solving computational problems. The condition of the problem and its solution are drawn up in this way. A sheet of notebook or blackboard is divided by a vertical line into two unequal parts. In the left, smaller part, the condition of the problem is abbreviated, a horizontal line is drawn and under it they indicate what needs to be found or calculated. Mathematical formulas, explanation, calculations and answer are written on the right side.

80 g of the compound contains 32 g oxygena. Calculate the mass fraction of oxygen in the compound.

The mass fraction of an element in a compound is also calculated using the chemical formula of the compound. Since the masses of atoms and molecules are proportional to the relative atomic and molecular masses, then

where N(E) is the number of element atoms in the compound formula.

From the known mass fraction of the element, it is possible to calculate the mass of the element contained in a certain mass of the compound. From the mathematical formula for the mass fraction of an element follows:

m(E) = w(E) m(compounds).

What mass of Nitrogen is contained in ammonium nitrate (nitrogen fertilizer) weighing 1 kg, if the mass fraction of this element in the compound is 0.35?

The concept of "mass fraction" is used to characterize the quantitative composition of mixtures of substances. The corresponding mathematical formula looks like this:

findings

The mass fraction of an element in a compound is the ratio of the mass of the element to the corresponding mass of the compound.

The mass fraction of an element in a compound is calculated from the known masses of the element and the compound or from its chemical formula.

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92. How to calculate the mass fraction of an element in a compound if: a) the mass of the element and the corresponding mass of the compound are known; b) the chemical formula of the compound?

93. 20 g of a substance contains 16 g of Bromine. Find the mass fraction of this element in the substance, expressing it as an ordinary fraction, decimal fraction and as a percentage.

94. Calculate (preferably orally) the mass fractions of elements in compounds with the following formulas: SO 2 , LiH, CrO 3 .

95. Comparing the formulas of substances, as well as the values ​​of relative atomic masses, determine in which of the substances of each pair the mass fraction of the first element in the formula is greater:

a) N 2 O, NO; b) CO, CO 2 ; c) B 2 O 3, B 2 S 3.

96. Perform the necessary calculations for acetic acid CH 3 COOH and glycerol C 3 H 5 (OH) 3 and fill in the table:

97. The mass fraction of Nitrogen in a certain compound is 28%. What mass of the compound contains 56 g of Nitrogen?

98. The mass fraction of Calcium in its combination with Hydrogen is 0.952. Determine the mass of hydrogen contained in 20 g of the compound.

99. Mixed 100 g of cement and 150 g of sand. What is the mass fraction of cement in the prepared mixture?

Popel P. P., Kriklya L. S., Chemistry: Pdruch. for 7 cells. zahalnosvit. navch. zakl. - K .: Exhibition Center "Academy", 2008. - 136 p.: il.

Knowing the chemical formula, you can calculate the mass fraction of chemical elements in a substance. element in substances is denoted by the Greek. the letter "omega" - ω E / V and is calculated by 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)?

Decision:

M r (H 2 O) \u003d 2 * A r (H) + 1 * A r (O) \u003d 2 * 1 + 1 * 16 \u003d 18

2) Calculate the mass fraction of hydrogen in water:

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

Rice. 1. Formulation of the solution of problem 1

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

1) Calculate the relative molecular weight of the substance:

M r (H 3 RO 4) \u003d 3 * A r (H) + 1 * A r (P) + 4 * A r (O) \u003d 3 * 1 + 1 * 31 + 4 * 16 \u003d 98

2) We 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 tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. "Chemistry, Grade 8" / 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. Grade 8” / 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 institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§15)

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

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