Question

When 3.44g of the mineral gypsum, CaSO4⋅xH2O, is heated to 128∘C, 2.90g of CaSO4⋅1/2H2O is obtained.

What is the value of x in the formula of gypsum?

Answer 1

If you heat up the material CaSO4 * xH2O,

the mass decreases by 3.44 g - 2.90 g = 0.54 g which is water. 0.54 g = 0.02997 moles water

CaSO4 * 1/2H2O has a molecular weight of 145.15 g/mole

2.9 g is then 0.01998 mole

Divide both by 0.01998

CuSO4 * 1/2H2O = 0.01998 mole/0.01998 mole = 1 mole

0.02997 mole / 0.01998 mole = 1.5

So, in heating up the CuSO4 * 1/2H2O the relative amount of water given off is 1.5 moles. Since there is still 0.5 moles of H2O left bound to the CuSO4, the original material was

CaSO4 * 2H2O

x = 2