If 9g of alumnium reacts with 35.5g of chlorine, what is the empirical formula of the compound formed?

Empirical formulaThe empirical formula is a formula giving the proportions of the elements present in a compound but not the actual numbers or arrangement of atoms. To find this you require the mass used and the relative mass numbers of the elements, which are found on the periodic table. It is easiest to set out the calculation as a grid to minimise mistakes: Al Grams used: 9 Mass number: 27 Grams used/mass number: 9/27 = 1/3 Cl Grams used: 35.5 Mass number: 35.5 Grams used/mass number: 35.5/35.5 = 1 Ratio of Al to Cl atoms 1 : 3

The calculation shows that for every 1 atom of aluminium there are 3 atoms of chlorine, therefore the empirical formula is AlCl₃.