How do you find the formula of a hydrate when given an impure substance?

Question

ow do you find the formula of a hydrate when given an impure substance? Is there any procedure to find this out in a laboratory setting?

Answer 1

The procedure to follow in a laboratory setting is to weigh the compound of interest and then heat it to above 100 degrees C (assuming it doesn't melt) to drive off all the water of hydration.  Then weigh the compound again, and continue to heat and weigh until the weight remains constant.  This suggests all the water has been eliminated. The difference in the initial weight and final weight represents the weight of water.  Moles of water = mass of water/18.

Answer 2

each molecule of water weighs 18 grams per mole so if you know the moles of your substance and it should weigh 20 grams for example but instead it weighs 56 grams, then you know there are two molecules of water in the hydrate