It is often possible to change a hydrate into an anhydrous compound by heating it to drive off the water (dehydration).

Question

It is often possible to change a hydrate into an anhydrous compound by heating it to drive off the water (dehydration).

A 44.97 gram sample of a hydrate of CaSO₄ was heated thoroughly in a porcelain crucible, until its weight remained constant. After heating, 35.56 grams of the anhydrous compound remained. What is the formula of the hydrate?

Answer 1

To find the formula of the hydrate, we need to find the moles of water that are attached. To do this, we will first find the grams of water and the convert to moles. We'll do the same thing for CaSO4.

molar mass CaSO4 = 136.1 g / mol

molar mass H2O = 18.02 g / mol

mass of H2O = 44.97 g - 35.56 g = 9.41 g H2O

moles of H2O = 9.41 g x 1 mol / 18.02 g = 0.522 mols H2O

mass CaSO4 = 35.56 g

moles CaSO4 = 35.56 g x 1 mol / 136.1 g = 0.261 mols CaSO4

Finding formula of the hydrate.

There are twice as many moles of H2O (0.522 mols) as there are mols of CaSO4 (0.261)

So we can write the formula as CaSO₄•2H₂O