Empirical Formula of a Hydrate Lab
Safety:
Crucibles are VERY HOT; always handle t hem 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:
A hydrate is a chemical that has water molecules loosely bonded to it. The water
molecules are not actually part of the formula, so the formula is written slightly
different ly. An example would be CaSO4 . 3H2O. This chemical would be called calcium
sulfate trihydrate. When finding the mass of this chemical, you find the mass of the
calcium sulfate and then add 3 times the mass of water to it. (40.08 + 32.066 + 4(15.999)
+ 3(2(1.0079) + 15.999)) = 190.19 g/mol.
The water can easily be removed from a hydrate just by heating strongly. You will be
weighing a hydrate and heating it to remove t he 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 t he H2O in the formula.
You will be using the hydrate CuSO4 .?H2O.
Sample CalculationAn 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: MgSO4 . ?H2O?
Mass of hydrate = 13.454 -12.770 = .684 grams
Mass of anhydrous salt = 13.010 - 12.770 = .240 grams
Mass of water = 0.684 -0.240 = .444 grams
Moles of anhydrous salt = .240 grams MgSO4 x 1 mol MgSO4 = .00199 moles MgSO4
120.367 g MgSO4
Moles of water = .444 grams H2O x 1 mol H2O = .0246 moles H2O
18.0148 g H2O
Ratio of moles of anhydrous salt to anhydrous salt = .00199/.00199 = 1
Ratio of moles of water to moles of anhydrous salt = .0246/.00199 = 12
Therefore the formula is MgSO4 . 12H2O
Pre_lab questions:
1. What is the formula for copper (II) sulfate?
2. What is the molar mass of copper (II) sulfate?
3. Use the following data to find the formula of the hydrate BeO . ?H2O
Mass of hydrate = 8.61 grams
Mass of water = 3.60 grams
What is the mass of the anhydrous salt?
What is the formula of the hydrate?
Procedure:
1. Clean and dry a crucible and find its mass on an accurate balance.
2. Obtain a scoop of the hydrate from your teacher and find the mass again.
3. Heat the crucible under moderate heat for 3-5 minutes. Let the crucible cool and find
its mass again.
4. Heat the crucible again under moderate heat for another 2 minutes. Find its mass again.
5. Repeat this heating and weighing until the mass doesn't change any more (stays within .05
grams).
6. Scrape your anhydrous salt into the disposal container and clean up your area.
Data:
Mass of empty crucible: ____________ g
Mass of crucible and hydrate ____________ g
Mass of crucible and anhydrous salt: _____________
Calculations:
Mass of water:
Mass of anhydrous salt:
Moles of Water:
Moles of Salt:
Formula of Hydrate: