Question

A 5.018 gram sample of a certain hydrate of magnesium sulfate, mgso4•xh2o, is heated until all the water is driven off. the resulting anhydrous compound weighs 2.449 grams. what is the formula of the hydrate?

Answer 1

Hydrate of magnesium sulphate undergoes dehydration upon action of heat. This process can be chemically represent as follows:

MgSO4.XH2O  → MgSO4 + XH20
       (5.018 g)      (2.449 g)     (2.569 g)

Number of moles of H2O evolved = $\frac{2.569}{Molecular.weight}$
                                                      = $\frac{2.569}{18}$
                                                      = 0.1427
Number of moles of MgSO4 generated after decomposition,
= $\frac{2.449}{Molecular.weight}$
=$\frac{2.449}{120.37}$
=0.0203

Now dividing  both these numbers by the smallest one to get the mole ratio that exists between MgSO4 and H20

For H2O, $\frac{0.1427}{0.0203}$ = 7.02 ~7

Thus, formula of hydrate is MgSO4.7H2O