A 1.013g sample of ZnSOsub4 * Hsub2O is dissolved in water and the sulfate ion is precipitated as BaSOsub4. The mass of pure dry BaSOsub4 obtained is 0.8223g. what is the formula of the zinc sulfate hydrate?
1 Answer

Molar mass of ZnSO4 = 161.472 g/mole
Molar mass of BaSO4 = 233.43 g/mole
Molar mass of H2O = 18 g/mole
1 mole of BaSO4 contains 1 mole of sulfate ions
Then 233.43g of BaSO4 will contain 96.06g of sulfate ions
Then, mass of sulfate ions in 1.013g of BaSO4 = 1.013×96.06/233.43 = 0.4169g
1 mole of ZnSO4 contains 1 mole of sulfate ions, and will be precipitated as 1 mole of BaSO4.
The number of BaSO4 moles precipitated = 0.8223/233.43 = 0.0035 moles
This means, there is also 0.0035 moles of ZnSO4 in this sample
Then, mass of ZnSO4 in sample = 0.0035×161.472 = 0.5688g
The, mass of water in 1.013g of ZnSO4 hydrate = 1.013 – 0.5688 = 0.4442g
The number of water moles in sample = 0.4442/18 = 0.0247 moles
Ratio of water to ZnSO4 moles in sample = 0.0247/0.0035 = 7 approx.
So for every mole of ZnSO4 there’s 7 moles of water of hydration bonded with it.
Then the formula of ZnSO4 hydrate is:
ZnSO4.7H2O (Zinc Sulfate Heptahydrate)