Question

copper sulfate pentahydrate, CuSo4 * 5 H2O. a 3.0000 g sample of the bright blue substance is strongly heated until a constant weight is obtained. the pale gray-white rsidue is found to weigh 1.6762 g. what is the actual formula for the copper sulfate hydrate.

Answer 1

Given the pale gray-white residue is CuSO₄ and the mass of CuSO₄ obtained is 1.6762 g

Molar mass of CuSO₄ is = 63.5 + 32 + (4x16) = 159.5 g/mol

Number of moles of CuSO₄ is n = mass/molar mass

                                                    = 1.6762 g / 159.5 (g/mol)

                                                    = 0.01 mol

Let the formula for the copper sulfate hydrate is CuSO₄ .aH₂O

Molar mass of CuSO₄ .aH₂O is = Molar mass of CuSO₄ + (a x molar mass of H₂O)

                                                   = 159.5 + (a x 18 )

                                                   = 159.5+18a

Given mass of sample is 3.0000 g

Number of moles of CuSO₄ .aH₂O is , n' = mass/molar mass

                                                                 = 3.0000 g / (159.5+18a) (g/mol)

CuSO₄ .aH₂O CuSO₄ + aH₂O

1 mole of CuSO₄ .aH₂O produces 1 mole of CuSO₄

0.01 mole of CuSO₄ .aH₂O produces 0.01 mole of CuSO₄

So number of moles of CuSO₄ .aH₂O = 0.01

               3.0000 / (159.5+18a) = 0.01

                        (159.5+18a) = 300

                                                     18 a = 140.5

                                                          a = 7.8

Therefore the actual formula for the copper sulfate hydrate is CuSO₄ 7.8 H₂O