Question

Plaster of Paris is the compound calcium sulfate hemihydrate, CaSO4*1/2H2O

What mass of water is liberated when a 2.763 gram sample is heated to constant mass?

The sample is 2.763 grams

How much of the 2.763 gram sample is water? I need an amount in grams.

Amount of anhyrdate + amount of water = amount of sample

Please help

Answer 1

You can do this the long way or a short way or one that's in between. The short way isn't taught anymore I don't think. The long way is to write an equation, balance it, and solve as if it were a stoichiometry problem. Here is a worked example that will solve all of your stoichiometry problems.
http://www.jiskha.com/science/chemistry/stoichiometry.html

The short way is
2.763g x (molar mass 0.5H2O/molar mass CaSO4.0.5H2O) = ? g H2O.

The in between way is to calculate the %H2O in CaSO4.1/2H2O.
That is (1/2 molar mass H2O/molar mass CaSO4.1/2H2O) x 100 = %H2O
Then (%H2O/100)*2.763 = g H2O (note the short way and the in between way are really the same thing but the approach is different)