Experiment 2
Gravimetric Determination of Water of Hydration

OBJECTIVES/OVERVIEW

In this experiment you will determine the percent water in a hydrate.

A hydrate is a solid that contains water in its crystalline lattice. Alum, KAl(SO4)2.12H2O and plaster of Paris, CaSO4.1/2H2O are examples of hydrates. The water molecules occupy regular lattice positions and are present in simple stoichiometric ratios. The theoretical percentage of water present in a hydrate can be easily calculated by dividing the mass of water per mol of hydrate by the molar mass (including water) of the hydrate (Equation 1).

Eq. 1

The water of hydration can often be driven off by mild heating leading to the anhydrous (without water) form of the substance as illustrated in Equation 2. The anhydrous forms of many solids are used as drying agents.

Eq. 2

The percentage of water of hydration in a given material can be experimentally determined by heating a weighed sample of the substance to drive off the water and reweighing to determine the mass of water lost. The mass loss divided by the mass of the original sample, multiplied by 100 (Equation 3) will give the percentage of water in the hydrate.

Eq. 3