A 2.914g sample of gypsum,a hydrated salt of calcium sulphate,CaSO4,Is heated at a temperature greater than 17C in a crucible until a constant mass is reached.

Question

The mass of anhydrous CaSO4 salt is 2.304 g . Calculate the percent by mass of water in hydrated calcium sulphate salt ???

Answer 1

I don't think you mean 17ºC until constant mass.  That temperature should probably read 170ºC.  It has to be >100ºC in order to evaporate the water of hydration.

Mass H2O of hydration = 2.914 g - 2.304 g = 0.610 g H2O

% H2O by mass = 0.610 g/2.914 g (x100%) = 20.93%