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%.

Thank you!

To determine the percent by mass of water in the hydrated calcium sulfate salt, you need to consider the following:

1. Begin by calculating the mass of water lost during heating:

Mass of water = Mass of hydrated salt - Mass of anhydrous salt
Mass of water = 1.994 g - 1.577 g
Mass of water = 0.417 g

2. Next, calculate the percent by mass of water in the hydrated salt:

Percent by mass of water = (Mass of water / Mass of hydrated salt) * 100
Percent by mass of water = (0.417 g / 1.994 g) * 100
Percent by mass of water = 20.91%

Therefore, your calculation is correct. The percent by mass of water in the hydrated calcium sulfate salt is indeed 20.91%.