A sample of a hydrate of barium chloride weighing 61 g was heated until all the water of hydration is removed. The dried sample weighed 52 g. The formula of the hydrated salt is : (atomic mass, Ba = 137 amu, Cl = 35.5 amu)

Let the formula of the hydrated salt be BaCl2.xH2O
BaCl2.xH2O Δ → BaCl2 + xH2O

Molar mass (g/mol) 208 + 18x 208
Mass (g) 61 52

Thus
208/61 = 52/(208+18x)
244 = 208 + 18x
36 = 18x
x = 2

Hence, the formula of the hydrated salt is BaCl2.2H2O.