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

The correct answer is MgSO₄.7H₂O.

The hydrate of magnesium sulfate goes through dehydration due to action of heat. This procedure can be represented chemically as follows:

MgSO₄.XH₂O (5.018 g) = MgSO₄ (2.449 g) + XH₂O (2.569 g)

The number of moles of H2O evolved = 2.569 / molecular weight

= 2.569 / 18 = 0.1427

Number of moles of MgSO₄ produced after decomposition,

= 2.449 / molecular weight

= 2.449 / 120.37

= 0.0203

Now dividing both these numbers to obtain the mole ratio, which prevails between MgSO₄ and H₂O

For H2O = 0.1427 / 0.0203 = 7.02 or 7

Therefore, the formula for the hydrate is MgSO₄.7H₂O.