GREEN CRYSTALLINE COMPOUND PART II: DETERMINATION OF
PERCENT WATER
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 K
x
Fe(C
2
O
4
)
y
.
zH
2
O. 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 (CaSO
4
.
2H
2
O) and Glauber’s Salt (Na
2
SO
4
.
10H
2
O) 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:
BaCl
2
.
2H
2
O(s) BaCl
2
(s) + 2H
2
O(g)
After the water has been driven off, anhydrous barium chloride remains.
The percentage of water of hydration in K
x
Fe(C
2
O
4
)
y
.
zH
2
O 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.
K
x
Fe(C
2
O
4
)
y
.
zH
2
O K
x
Fe(C
2
O
4
)
y
(s) + zH
2
O(g)
The loss in mass corresponds to the mass of the water of hydration.
Prelab Problem: A 1.47 gram sample of a calcium chloride hydrate compound
(CaCl
2
.
zH
2
O) 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?
