A student is given a sample of CaSO4 , which has two different hydrates:
-gypsum, or calcium sulfate dihydrate, CaSO4•2H2O
-plaster of paris. or calcium sulfate hemihydrate, (CaSO4)2•H2O
(This can also be written as CaSO4 • ½H2O, hence the “hemi” in the name. There is NOT ½
a molecule of water. It is just another way of noting the 2:1 ratio of CaSO4 to H2O.)
Due to time constraints, the student was instructed to only go through one heat-cool-weigh
cycle on the clean, empty crucible and cover. After heating and cooling her crucible, she
finds that the crucible and cover weighs 23.1342 g. She then loads her sample into the
crucible, and finds that the crucible, cover, and sample weigh 24.3046 g. First, she gently
heats the sample with the cover slightly ajar so as not to splatter material out as the water of
hydration is being driven off. Then, with the sample basically dehydrated, she heats the
crucible to red heat for about 10 minutes. After the crucible has cooled, she finds it has a
lower mass. The crucible, cover, and contents after heating and cooling weigh 24.0589 g.
To see if all of the water of hydration was driven off, she heats the sample again. This time,
she does not need to heat gently, since the water of hydration has probably all been driven
off already. After heating to red heat for another 5 minutes, she finds the crucible, cover,
and contents after cooling weigh 24.0593 g. Since both measurements rounded to 24.059 g
(i.e. reproducible to the 0.001 g), she is satisfied that the sample had been heated to constant
weight, and she reports the average of the crucible, cover, and contents as
(24.0593+ 24.0589)/2 g = 24.0591 g
Show all work in the spaces provided.
a. What was the mass of the original sample, the hydrated CaSO4? g
hydrated CaSO4
b. How much water was driven off? g H20
c. The MW of water is 18.02 g/mol.
How many moles of water were in the hydrate? moles H20
d. What is the mass of the anhydrous CaSO4? g CaSO4
e. The MW of CaSO4is 136.15 g/mol.
How many moles of anhydrous CaSO4 were in the hydrate? moles CaSO4
f. Calculate the ratio of the number of moles of CaSO4 to the number of moles of H2O, showing
the correct number of s.f., then rounding to the nearest integer or small fraction.
moles CaSO4 : moles H20 _____
g. What is the formula of the hydrate? ____
Is it gypsum or plaster of paris? ____
- If the formula of a sample is not known, important information can still be obtained,
since the percent water in a hydrate is a fixed quantity. In the case of the hydrates of
calcium sulfate, gypsum has 20.93% water.
Sample calculation: In 1 mole of gypsum, there are 2 moles of water.
2 H20 CaSO4
One mole of the hydrate weighs: (2 x 18.02 g) + 136.15 g = 172.19 g
Two moles of water weigh: (2 x 18.02 g) = 36.04 g
36.04 g / 172.19 g = 0.2093 or 20.93% water in gypsum
a. Calculate the theoretical percent water in plaster of paris. %
b. Use the data in 1a and 1b to calculate the experimentally determined percent water in
the sample. Experimental data is rarely exactly equal to the theoretical value, but it
should be close, and consistent with you identification of the hydrate (gypsum or
plaster of paris) in question 1g.
%
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