A hydrate of copper(II) sulfate is 25.5% copper by mass. How many water molecules are there for each unit of copper(II) sulfate?

Question

Quizplus · Accepted Answer

The formula for copper(II) sulfate is CuSO4. The molar mass of CuSO4 is the sum of the atomic masses of its constituent atoms: Cu (63.55 g/mol) + S (32.07 g/mol) + 4*O (4*16.00 g/mol) = 159.62 g/mol. Given that copper makes up 25.5% of the hydrate by mass, the mass of copper in 100 g of the hydrate would be 25.5 g. This corresponds to 25.5 g / 63.55 g/mol = 0.401 mol of Cu. Since the ratio of Cu to CuSO4 in the hydrate is 1:1, there are also 0.401 mol of CuSO4 in 100 g of the hydrate. The remaining mass in the 100 g of the hydrate is from water, which is 100 g - 25.5 g (Cu) - 63.96 g (CuSO4) = 10.54 g of water. The molar mass of water is 18.015 g/mol, so there are 10.54 g / 18.015 g/mol = 0.585 mol of water. The ratio of moles of water to moles of CuSO4 is 0.585 mol H2O / 0.401 mol CuSO4 ≈ 5:1, indicating there are 5 water molecules for each unit of copper(II) sulfate, making the correct answer E (5).

A Hydrate of Copper(II) Sulfate Is 25