empirical formula

Question

1.2 g of a sulphide of iron burns in excess oxygen gas to give 480 cm^3 of sulphur dioxide, measured at room temperature and pressure. what is its empirical formula?

Answer 1

Molar volume of gas at room condition = 24000 cm3
Relative atomic masses : O = 16.0, S = 32.1, Fe = 55.9
Molar mass of SO2 = 32.1 + 16x2 = 64.1 g mol-1

No. of moles of SO2 = vol / (molar vol) = 480/24000 = 0.02 mol
Mass of SO2 = mol x (molar mass) = 0.02 x 64.1 = 1.28 g
Mass of S in SO2 = 1.28 x (32.1/64.1) = 0.641 g

In 1.2 g of the sulphide of iron :
Mass of S = Mass of S in SO2 = 0.641 g
Mass of Fe = 1.2 - 0.641 = 0.559 g

Mole ratio Fe : S
= (0.559/55.9) : (0.641/32.1)
= 0.01 : 0.02
= 1 : 2
Hence, empirical formula = FeS2