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A 1.994 g sample of gypsum, a hydrated salt of calcium sulfate, CaSO4, is heated at a temp. above 170 C in a crucible until a constant mass is reached. The mass of anhydrous CaSO4 salt is 1.577 g.

Calculate the percent by mass of water in the hydrated calcium sulfate salt.

This is what I did....

1.577/1.944 * 100 = 79.09%

100-79.09 = 20.91% water.

Is this correct? Thanks for your help!

Yes, it's correct. Another way you could have done it (but no more correct than the way you did it) is
1.994 g = hydrated CaSO4
-1.577 g = anhydrous material
-----------
0.417 g = mass water driven off

%H2O = (0.417/1.944)*100 = 20.913 which I would round to 20.9%.

• Chemistry. -

Thank you!

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