Determine the formula of the hydrate.

Question

A hydrate of barium chloride, BaCl2nH2O, weighing 1.222 g was heated until all the combined water was expelled. The dry powder remaining weighed 1.042 g.

Determine the formula of the hydrate.

Answer 1

First, we find the mass of the water of hydration by determining mass of water lost on heating:

mass H₂O = 1.222 g - 1.042 g = 0.18 g H₂O

Next, we convert the mass of H₂O to moles of H₂O:

mols H₂O = 0.1800 g H₂O x 1 mol H₂O / 18 g = 0.01 mols H₂O

Next, we determine the mols of anhydrous (without water) BaCl₂:

molar mass anhydrous BaCl₂ = 208.2 g / mol

moles BaCl₂ = 1.042 g x 1 mol / 208.2 g = 0.00500 mols BaCl₂

Thus, we have determined that we have 0.01 mols H₂O and 0.0050 mols BaCl₂. The mol ratio is therefore

2 mols H₂O : 1 mol BaCl₂. This tells us the formula of the hydrate:

Formula of hydrate = BaCl₂ • 2H₂O