Formula of a Hydrate

Safety:
Crucibles are VERY HOT; always handle them with tongs. DO NOT put hot crucibles on a balance! They cost $9.00 if you
break it. Crucibles are VERY FRAGILE. Never carry them around without a heat-proof pad under it. Wear goggles at all
times.

Introduction:
The water can easily be removed from a hydrate just by heating strongly. You will be weighing a hydrate and heating it to
remove the water (now called "anhydrous salt") and weigh it again. You can now find the percent of the anhydrous salt and the
water. By finding a mol ratio, you can find out how many moles of water there are per mol of anhydrous salt. This number goes
just before the H2O in the formula.

Sample Calculation-
An empty crucible has a mass of 12.770 grams. The crucible and hydrate have a mass of 13.454 grams. After heating, the
crucible and anhydrous salt have a mass of 13.010 grams. What is the formula of this hydrate of
MgSO4 . ?H2O?
Mass of hydrate = 13.454 - 12.770 = .684 grams
Mass of anhydrous salt = 13.010 - 12.770 = .240 grams
Mass of water = 13.454 - 13.010 = .444 grams
 
 
Moles of anhydrous salt = .240 grams MgSO4 x 1 mol MgSO4 = .00199 moles MgSO4
1 120.367 g MgSO4
Moles of water = .444 grams H2O x 1 mol H2O = .0246 moles H2O
1 18.0148 g H2O
 
 
Ratio of moles of water to moles of anhydrous salt = .0246/.00199 = 12
Therefore the formula is MgSO4 . 12H2O