Question

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

Answer 1

We determine the percent by mass of water in the compound by dividing the mass of  water by the total mass. The total mass of Na2SO4.10H2O is equal to 322 g. The mass of water is 180 g. 
       
                    percent by mass of water = (180 / 322)*(100 %) = 55.9%

Answer 2

55.92 percent is the correct answer for online chemistry I just took the test!