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

Question

$\begin{array}{1 1} BaCl_2.H_2O \\ BaCl_2.2H_2O \\ BaCl_2.3H_2O \\ BaCl_2.4H_2O \end{array}$