You will need
• You need to determine which option applies to your task. In the case of the first option you will need a periodic table. In the case of a second - it is necessary to know that the solution consists of two components: the solute and solvent. And the mass of solution is equal to the masses of these two components.
Instruction
1
In the case of the first variant of the exercise:
On the periodic table find the molar mass of the substance. Molar mass is equal to the sum of the atomic masses of the elements included in the composition of the substance.

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

Molar mass of atoms are taken from the periodic table.
2
The calculated mass fraction of element (ω), for example, calcium in calcium hydroxide.

Mass fraction equal to the ratio of the atomic mass of the element by the molar mass of the substance: ω = Ar:Mr.

In the case of calcium hydroxide: ω(CA) = 40:74 = 0,54. Is mass part of element in fractions of a unit.
3
In the case of the second version of the problem:
To determine what mass you are given, namely: the mass of solute and mass of solution or mass of solute and mass of solvent.
4
If a given mass of solute and solution, the mass fraction is the ratio of mass of dissolved (R. V.) of a substance to the mass of the solution (R-RA).

ω = m(R. V.):m(R-RA)

For example, if the mass of salt 40 g and mass of solution is 100 g, then ω(salt) = 40:100 = 0,4. Is the mass fraction of the solute fraction.
5
If a given mass of solute and solvent, it is first necessary to determine the mass of the solution. The mass of the solution (R-RA) is equal to the sum of the masses of solute (R. V.) and solvent (R-La).

m(R-RA) = m(RV) + m(R-La)

For example, if the mass of salt 40 g and mass of water 60 g, then m(R-RA) = 40 + 60 = 100 (d).

Then we can calculate the mass fraction of the solute similar to the previous step.
6
To find the mass percentage you need to multiply the mass fraction in parts per 100.

ω(Sa) = of 0.54 * 100 = 54 %