Mass % and mole% of elements in compounds

Formula for substance to moles to mole fraction or mole% 

The subscripts in the formula for a substance indicate

• the relative numbers of the atoms in one formula unit of the substance, and
• the relative amounts in moles of those atoms in one mole of the substance.

molecular composition	1 molecule of H2O contains	2 atoms of H	and 1 atom of O
amount in moles to	1 mole of H2O	2 moles of H	and 1 mole of O		divide each by the total number of atoms
mole fraction to	The mole fractions for H2O	0.67 H	0.33 O		multiply by 100%
mol percent %	Any mass of H2O has	67 mole% H	33 mole % O

 Formula for substance to moles to grams to mass %

The subscripts in the formula for a substance indicate

• the relative numbers of the atoms in one formula unit of the substance, and
• the relative amounts in moles of those atoms in one mole of the substance.

The molar mass of each element in the compound can then be used to calculate the mass of each element in the compound.
This in turn can be used to calculate the mass percentage of each element in the compound.

molecular composition	1 molecule of H2O contains	2 atoms of H	and 1 atom of O		m = M × n
amount in moles to	1 mole of H2O	2 moles of H	and 1 mole of O		multiply by the molar mass
mass in g to	18 g of H2O contains	2 g H	and 16 g of O		divide the mass of each element by the total mass
% by mass	Any mass of H2O has	11.1 mass % H	and 88.9 mass % O