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# How to determine the mass fraction

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There are two options for setting the problem: 1) when it is necessary to determine the mass fraction of an element in a substance; 2) when it is necessary to determine the mass fraction of the solute.

You will need

• It is necessary to determine to which option theyour task. In the case of the first option, you will need a periodic table. In the case of the second one, one must know that the solution consists of two components: a dissolved substance and a solvent. And the mass of the solution is equal to the masses of these two components.

Instructions

1

In the case of the first version of the problem:
According to Mendeleyev's table, we find the molar mass of the substance. The molar mass is equal to the sum of the atomic masses of the elements that make up the substance.

For example, the molar mass (Mr) of the calcium hydroxide Ca (OH) 2: Mr (Ca (OH) 2) = Ar (Ca) + (Ar (O) + Ar (H)) * 2 = 40 + (16 + 1) * 2 = 74.

Molar masses of atoms are taken from the periodic table.

2

Calculate the mass fraction of the element (?), For example calcium in calcium hydroxide.

The mass fraction is equal to the ratio of the atomic mass of the element to the molar mass of the substance:? = Ar: Mr.

In the case of calcium hydroxide:? (Ca) = 40: 74 = 0.54. This is the mass fraction of the element in fractions of one.

3

In the case of the second version of the problem:
Determine what masses are given to you, namely: the mass of the dissolved substance and the mass of the solution or the mass of the dissolved substance and the mass of the solvent.

4

If the masses of the dissolved substance and solution are given, the mass fraction is equal to the ratio of the mass of the dissolved (re) substance to the mass of the solution (p-ra).

? = M (rv): m (p-ra)

For example, if the mass of the salt is 40 g, and the mass of the solution is 100 g, then? (Salt) = 40: 100 = 0.4. This is the mass fraction of the solute in fractions of one.

5

If the masses of the dissolved substance and the solvent are given, then the mass of the solution must first be determined. The mass of the solution (p-ra) is equal to the sum of the masses of the solute (rv) and the solvent (p-la).

M (p-pa) = m (rv) + m (p-la)

For example, if the mass of the salt is 40 g and the mass of water is 60 g, then m (p-pa) = 40 + 60 = 100 (g).

Then the mass fraction of the solute is calculated in the same way as the previous step.

6

To find the mass fraction in percentage, you must multiply the mass fraction in fractions of a unit by 100.

? (Ca) = 0.54 * 100 = 54%

How to determine the mass fraction Was last modified: June 21st, 2017 By
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