Question

A powder contains FeSO4·7H2O (molar mass = 278.01 g/mol) among other components. A 3.285-g sample of the powder was dissolved in HNO3 and heated to convert all iron to Fe3 . The addition of NH3 precipitated Fe2O3·xH2O which was subsequently ignited to produce 0.438 g of Fe2O3.

What was the mass of FeSO4·7H2O in the 3.285-g sample?

Answer 1

2 FeSO4·7H2O ------------------------> Fe2O3

moles of FeSO4·7H2O = 2 x moles of Fe2O3

                                       = 2 x 0.438 / 159.69

                                    = 5.49 x 10^-3

mass of FeSO4·7H2O = 5.49 x 10^-3 x 278.01

                                      = 1.525 g

mass of FeSO4·7H2O = 1.525 g