you need
  • periodic table, calculator.
Guide
1
determine the chemical form of the substance, the mass fraction of the elements that you want to find.Take Mendeleev's periodic system, and see in it the elements of the cell corresponding to the atoms in the molecule of the substance.In the cell, find the mass number of each such element .If the value found for the mass number of the element fractional, round it to the nearest integer.
2
In the case where the atoms of the same type found in the molecule several times, multiply by the number of atomic mass.Slozhimte weight of all the elements that make up the molecule to get the value in atomic ma
ss units.For example, to find a lot of salt molecules, called sodium sulfate (Na2SO4), determines the atomic weight of sodium Ar (Na) = 23, S Ar (S) 32 = Ar and oxygen (O) = 16.Since the molecule contains 2 atoms of sodium, then it take the value of 2 * 23 = 46, and an oxygen atom in which 4 - 16 * 4 = 64.Then the mass of the molecule will be MR sodium sulfate (Na2SO4) = 46 + 32 + 64 = 142.
3
To calculate the mass fractions of the elements that make up the molecule of the substance, find the ratio of the masses of atoms in a molecule of a substance to the mass of the molecule itself, and multiply the result by 100%.For example, if we consider the sodium sulphate Na2SO4, calculate the mass fractions of elements such as: - the mass fraction of sodium amount ω (Na) = 23 • 2 • 100% / 142 = 32.4%;
- mass fraction of sulfur will be ω (S) = 32 • 100% / 142 = 22.5%;
- mass fraction of oxygen will be ω (G) = 16 • 4 • 100% / 142 = 45.1%.
4
Mass share show the relative abundance of the elements in the molecule of the substance.Check the calculations, the mass fraction of folded material.They will amount to 100%.In this example, 32.4% + 22.5% + 45.1% = 100%, then the calculation is made correctly.