Gypsum is hydrated calcium sulfate. a 4.89 g sample of this hydrate was heated. after the water was removed, 3.87 g anhydrous calcium sulfate remained. determine the formula of this hydrate and name the compound.

Answer is: calcium sulfate dihydrate (CaSO₄ · 2H₂O)
m(CaSO₄ · xH₂O) = 4,89 g.
m(CaSO₄) = 3,87 g.
n(CaSO₄) = m(CaSO₄) ÷ M(CaSO₄)
n(CaSO₄) = 3,87 g ÷ 136 g/mol = 0,0285 mol.
m(H₂O) = m(CaSO₄ · xH₂O) - m(CaSO₄).
m(H₂O) = 4,89 g - 3,87 g 
m(H₂O) = 1,02 g.
n(H₂O)  = m(H₂O) ÷ M(H₂O) 
n(H₂O)  = 1,02 g ÷ 18 g/mol
n(H₂O) = 0,057 mol.
n(CaSO₄) : n(H₂O) = 0,0285 mol : 0,057
n(CaSO₄) : n(H₂O) = 1 : 2