Question

On heating 5.00g of a fresh sample of X to constant mass, 1.80g was lost in the form of water vapour. Calculate the number of molecules of water crystallization in one molecule of X. (O=16, H=1, anhydrous form of X=160g/mol)​

Answer 1

The mass of water lost from the sample of X is 1.80g. The molar mass of water is (2 x 1) + 16 = 18 g/mol. Therefore, the number of moles of water lost is 1.80g / 18 g/mol = 0.1 mol.

The mass of the anhydrous form of X is 5.00g - 1.80g = 3.20g. The molar mass of the anhydrous form of X is given as 160 g/mol. Therefore, the number of moles of anhydrous X is 3.20g / 160 g/mol = 0.02 mol.

Since the number of moles of water lost is 0.1 mol and the number of moles of anhydrous X is 0.02 mol, there are (0.1 mol / 0.02 mol) = 5 molecules of water crystallization in one molecule of X.