Find the percent anhydrate in CaSO4 - 2H2O. If 16g of the calcium sulfate hydrate are heated until all the water evaporates off, mass?

Molar mass of CaSO₄ = (40.1 + 32.1 + 16.0×4) g/mol = 136.2 g/mol
Molar mass of CaSO₄•2H₂O = (136.2 + 1.0×4 + 16.0×2) g/mol = 172.2 g/mol

Percent anhydrate in CaSO₄•2H₂O = (136.2/172.2) × 100(%) = 79.1%

When CaSO₄•2H₂O are heated until all water evaporates off :
Mass remained = (16 g) × (136.2/172.2) = 13 g (to 2 sig. fig.)

Molar mass of hydrate = 172.17 g/mol
Mass CaSO4 = 136.14 g/mol

Mass remaining = 16 g X (136.14 / 172.17) = 12.65 g

% mass remaining = (12.65 / 16) X 100 = 79%