Formula of a Hydrate Lab
Safety:
Crucibles are VERY HOT; always handle them with
tongs. Let Crucibles cool before weighing. Crucibles are VERY
FRAGILE. 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 differently. 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
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.
You will be using the hydrate CuSO4 . ?H2O.
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 crucible and hydrate – mass of crucible = Mass of hydrate
13.454 - 12.770 = .684 grams
Mass of crucible and anhydrous salt-mass of crucible = Mass of
anhydrous salt
13.010 - 12.770 = .240 grams
Mass of crucible and hydrate – mass of crucible and anhydrous salt
= Mass of water
13.454 - 13.010 = .444 grams
Moles of anhydrous salt =
.240 grams MgSO4 X 1 mol/120.36g = .00199 moles MgSO4
Moles of water =
.444 grams H2O x 1 mol/18.02g = .0246 moles H2O
Ratio of moles of water to moles of anhydrous
salt=.0246/.00199=12
Therefore the formula is MgSO4 . 12H2O
Procedure:
1. Heat a crucible and find its mass on a balance.
2. Crush some copper(II)sulfate. Add to your crucible. Find the
mass again. (Should be around 15 grams more than your previous
measurement)
3. Heat the crucible under moderate heat for 3-5 minutes. Make
sure the lid is on, but cracked to let evaporated water escape. Let
the crucible cool and find it’s mass again.
4. Heat the crucible again under moderate heat for another 2
minutes. Find it’s mass again.
5. Repeat this heating and weighing until the mass doesn't change
any more (stays within .1 grams).
6. Record all of the masses until there is no more water left.
7. 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 trial 1 ____________ trial 2
______________ trial 3 (if needed) _______________
Calculation:
Mass of water _____________
Mass of anhydrous salt _____________
Molar mass of Copper(II) Sulfate_____________
Molar mass of Water _______________
Moles of Copper(II)Sulfate_____________
Moles of Water_______________
Ratio of Moles of Water to Copper(II)Sulfate_______________
Formula for the Copper(II)Sulfate hydrate________________