Question

What is the percent by mass of water in hydrated gypsum, CaSO₄•2H₂O? Notice that there are 2 moles of H₂O for every mole of CaSO₄.

Answer 1

To determine the percent of water, we have to divide the mass of water with the total mass of the compound and then we have to multiply with 100.

The total molar mass of the compound CaSO₄.2H₂O = 172.1712 g

molar mass of water = 18.01528 g/mol

We have to moles of water, therefore,
mass of water = 2 x 18.01528 g/mol
              = 36.1056 g

Therefore, the mass % of water = (mass of water/ mass of CaSO₄.2H₂O)x100
                               = (36.1056 g/172.1712 g)x100
                               = 20.97 %

Hence, the percent of water in Gypsum = 20.97 %