Question

Calcium sulfate is a white solid found as two hydrates, a hemihydrate known as plaster of Paris and a dehydrate known as gypsum. The hemihydrate is a white solid. Given that the molar mass of the anhydrous calcium sulfate is 136.14 g/mol, the molar mass of the hemihydrate is 145.15 g/mol, and the molar mass of water is 18.015 g/mol, which formula gives the correct ratio for the hemihydrate

Answer 1

2(CaSO4)*H2O

Step-by-step explanation:

To solve this question we need to find the moles water, H2O, of calcium sulfate, CaSO4 as follows:

The molar mass of the anhydrous CaSO4 are 136.14g/mol and the molar mass of the hemihydrate is 145.15g/mol. The differences in molar mass represents the additional mass that water is producing:

145.15g/mol - 136.14g/mol = 9.01g/mol

A 1 mole of water weighs 18.015g/mol, the additional mass of water is due the addition of 1/2 moles of H2O. That means we have 1 mole of CaSO4 per 1/2 mole of H2O. That is the same:

2(CaSO4)*H2O

And this is the formula that gives the correct ratio for the hemihydrate.