Compound amounts and percentage composition

Calculate molar mass by adding atomic molar masses with the multipliers shown in the formula. One mole of CaCl2​ formula units contains one mole of Ca2+ ions and two moles of Cl− ions. Formula units, molecules and individual ions are different entities, so specify what an amount counts. The relation n=Mm​ applies to compounds as well as elements.

The mass percentage of an element equals its contribution to formula mass divided by total formula mass, multiplied by 100. To find an empirical formula from composition, convert each element mass to moles, divide by the smallest amount and obtain the simplest whole-number ratio. A molecular formula may be a whole-number multiple of the empirical formula, identified using the molecular molar mass.

Watch out: Each formula unit has two chloride ions, so one mole of CaCl2​ contains two moles of Cl−.