Question

What is the percent by mass of water in Na2SO4 • 10H2O?

Answer 1

M = M Na2SO4 + 10 M H2O
M = ((2 * 23 + 32 + 4 * 16) + 10 * 18) g/mole
M = (46 + 32 + 64 + 180) g/mole
M = 322 g/mole

So:
In 322 g Na2SO4 * 10H2O you have 180 g H2O
In 100 g Na2SO4 * 10H2O you have p g H2O

p is the mass percent of H2O in the Na2SO4 * 10H2O

p = (100 * 180)/322 = 55.90% H2O