posted by Roy .
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?
Consider the reaction:
CaSO4*(1/2H2O) -> CaSO4 + 1/2H2O
1/2 mole of H2O will be released per mole of CaSO4*(1/2H2O)
Start by calculating how many moles of CaSO4*(1/2H2O) you start with.
The molar mass of CaSO4*(1/2H2O) is
40 + 32 + 64 + 9 = 145 g/mol
You start out with 0.01906 moles of the hemihydrate and end up with half as many moles of H2O.