1.33 calculate empirical and molecular formulae from experimental data

Calculating Molecular Formula

1	Write the element symbols	S	O
2	Write the masses or the % of the elements	3.2 g	6.4 g – 3.2 g = 3.2 g
3	Write the Ar values of each element	32	16
4	Divide masses by At	3.2 ÷ 32 = 0.1	3.2 ÷ 16 = 0.2
5	Divide by the smallest number	0.1 ÷ 0.1 = 1	0.2 ÷ 0.1 = 2
6	Write the formula	SO2

Calculating Molecular Formula

From the empirical formula, you can work out the molecular formula from the relative formula mass (M_r) of the compound.

• Add up the atomic masses of the atoms in the empirical formula.

For example, the empirical formula of a hydrocarbon is CH₂ and its M_r is 42.

• The mass of the atoms in the empirical formula is 14
• 42 ÷ 14 = 3
• so you need to multiply the numbers in the empirical formula by 3

The molecular formula of the hydrocarbon is therefore C₃H₆.