Determine the molecular formula of the hydrate.

Question

A 2.78 g sample of hydrated iron(II) sulfate was heated to remove all of the water of hydration. The mass of the anhydrous iron(II) sulfate was 1.52 g. Determine the molecular formula of the hydrate.

Answer 1

The mass of water is obtained by subtracting the mass of the compound after heating from the mass before heating.

2.78 - 1.52 = 1.26 g which is the mass of the water in the compound

The number of moles of the water is given by 1.26 g / 18 g/mole = 0.07 moles.

The number of moles of anhydrous FeSO4 is 1.52 g / MM FeSO4 = 1.52 / 152 g/mole = 0.01 moles FeSO4.

The ratio of moles water to moles FeSO4 is 0.07 / 0.01 or 7/1.

Therefore, the molecular formula of the hydrate is FeSO4 • 7H2O.