GREEN CRYSTALLINE COMPOUND PART II: DETERMINATION OF
PERCENT WATER
PURPOSE: This is the second in a series of four experiments in which you will
synthesize and analyze a green crystalline compound having the general formula
KxFey(C2O4)z · a H20. In this experiment you are to determine the percent water in the
compound. The green crystalline compound is a hydrate compound. A hydrate is a
compound that contains water that is chemically bound in the solid state, and is present in
stoichiometric amounts. Plaster of Paris (CaSO4 · 2H2O), Glauber’s Salt
(Na2SO4 · 10 H2O) and concrete are familiar examples of hydrates. The water of
hydration of many hydrates can be driven off as gaseous water by heating the hydrate to a
temperature above 100ºC for a period of time. Barium chloride dihydrate loses its water
of hydration when heated above 100ºC as shown following:
BaCl2 · 2H2O(s) --------------> BaCl2(s) + 2H2O(g)
After the water has been driven off, anhydrous barium chloride remains. The percentage
of water of hydration in KxFe(C2O4)y .zH2O will be obtained in this experiment by
heating a massed sample of the green hydrate compound in an open container in an oven
until all of the water of hydration has been driven off.
KxFe(C2O4)y · z H20 -----------------> KxFe(C2O4)y(s) + z H2O(g)
The loss in mass corresponds to the mass of the water of hydration.
PRELAB QUESTIONS:
1. A 1.47 gram sample of a calcium chloride hydrate compound (CaCl2 · zH2O) is
heated until all the water is driven off. The sample of the anhydride compound that is left
has a mass of 1.11 grams. What percentage of the calcium chloride hydrate compound is
water? What is the value of z?
MATERIALS:
2-50 mL beakers
crucible tongs
drying oven
digital balance
2 crucibles
Iron complex crystal samples
PROCEDURE:
1. Before starting this experiment, the green crystalline compound should have been dried
at room temperature in your lab drawer for several days, massed and stored in a brown
bottle with a cap on it.
2. Obtain two 50 mL beakers and put a label containing your name on each one. Put a
crucible in each beaker and place in the oven at 100-120 ºC. Handle the crucibles with
tongs.
3. Allow the crucibles and beakers to cool for twenty minutes. Convection currents
created when hot objects are put on a balance can affect mass measurements.
4. Determine the masses of the empty crucibles and record these and all other masses in
this experiment to the nearest 0.001g.
5. Transfer between 1.000 and 1.200 grams of the green crystalline compound into each
crucible. Mass the crucibles and crystals, and use the same balance you used in
measuring the masses of the empty crucibles.
6. Record the masses of the crucibles and crystals, and subtract the masses of the
crucibles in each sample to get the masses of the crystals before they are heated.
7. Put the crucibles containing the crystals into the 50 mL beakers and heat for 45
minutes in the oven at 110-120 ºC. Do not leave the crystals in the oven for a longer
period of time or heat at a higher temperature, or the compound may decompose and turn
brown.
8. Take the beakers, crucibles and crystals out of the oven and let them cool. Mass the
crucibles and crystals and record the masses on your data sheet.
9. Return the crucibles with the green crystalline compound to the oven and heat for 30
more minutes. Follow this by cooling and massing. Repeat this process until the crucibles
and remaining crystals have a constant mass, or until the class period is complete.
10. Calculate the decrease in mass upon heating for each sample and attribute this loss in
mass to the water of hydration. Obtain the percent water of hydration for each sample,
and compute the average for the two samples.
11. After the final massing, discard the anhydrous salt in the waste container, clean the
crucibles, and put the brown bottles with your crystals (and caps on them) back into the
lab drawer and lock.
12. YOU WILL BE GRADED ON THE ACCURACY OF YOUR AVERAGE
PERCENT OF WATER.
DATA AND CALCULATIONS:
Include the following for both samples 1 and 2 in the data portion of your lab write
up:
Mass of crucible, mass of crucible and crystals before heating, mass of crystals before
heating, mass of crucible and crystals after first heating, mass of crucible and crystals
after second heating, mass of water, percent of water, average percent of water
CONCLUSIONS:
1. Why is it essential to heat the beakers and crucibles before actually adding the crystal
to them for subsequent heatings?
2. Why should successive heatings be done on the green crystalline compound in this
experiment until the compound comes to a constant mass?
3. What is the greatest contributor to experimental error in this laboratory? How could
we have corrected this?
4. Were there any visual indications that your hydrate did in fact lose water?