Question

given that the molar mass of Na2SO4 × nH2O is 322.1 g/mol calculate the number of moles of water in this hydrate

Answer 1

The molar mass of anhydrous Na₂SO₄ = [2(23) + 32 + 4(16)]
                                                                = 142 g/mol

If the molar mass of hydrated Na₂SO₄ = 322 g/mol

Since hydrated salt = Anhydrous Salt + Water

Then the mass of hydration (Water)= 322 g/mol - 142 g/mol    
                                                           = 180 g/mol

Since the molar mass of water itself is 18 g/mol
⇒ Moles of water = 180 g/mol ÷ 18g/mol
                              = 10

Thus n = 10; the formula of the hydrated salt is Na₂SO₄ · 10H₂O (Sodium Sulphate Decahydrate)