Express your answer to three significant figures.
If you could also thoroughly explain how to get the answer, that would be great!
Molar mass of MgSO4.7H2O =246.52 g/mol
Now 246.52 g/mol of MgSO4.7H2O contains 7×18 i.e. 126 g of H2O
So 31.0 g of MgSO4.7H2O will contain 31.0 x 126/246.52 = 15.8 g H2O
Thus weight of anhydrous MgSO4 = 31.0 – 15.8 = 15.2 g