If 20.0g of MgSO4⋅7H2O is thoroughly heated, what mass of anhydrous magnesium sulfate will remain?

We know that:

MgSO4.7H2O + heat = MgSO4 + 7 H2O

Also,

molar mass of MgSO4*7H2O = 246.47 g/mole

molar mass of MgSO4 (anhydrous) = 120.415 g/mole

Therefore,

Remaining mass of anhydrous magnesium sulfate = (20 grams of MgSO4⋅7H2O) / (246.4746 grams of MgSO4⋅7H2O/mole) x (1 mole of MgSO4 / 1 mole of MgSO4⋅7H2O) x (120.3676 grams of MgSO4/mole)

= 7.7 grams of MgSO4