Lab 2: Determine the Percentage of Water in a Hydrate

Purpose: The purpose of this experiment is to determine the water content of a hydrated salt.

Question: What is the water content of a hydrated salt?

Background:

Many naturally occurring or manmade salts contain water molecules bound within the crystal structure of the solid.  These are called hydrated salts.  The water molecules are known as "water of crystallization" or "water of hydration."

The number of moles of water will often remain in a fixed ratio to the number of moles of salt present.  The formula for a hydrated salt is written as the formula of the anhydrous(without water) salt followed by a raised dot followed by the number of water molecules.  For example, the formula for cobalt chloride hexahydrate is 

In some cases the water is loosely bound to the salt, allowing the water to be removed by applying heat:

Some salts have their water bound so tightly that producing an anhydrous salt is nearly impossible.  In the case of iron trichloride hexahydrate, the salt will decompose before all the water can be removed.

The percentage of water, by mass, in a hydrate can be determined by heating a known quantity until complete dehydration is achieved.

Dehydration results in decreased mass.  The difference of the mass before and after heating makes it possible to determine the amount of water that was present in the hydrate.

Total mass of hydrated salt= mass of anhydrous salt + mass of water of hydration

The percent mass of water in the hydrated salt is easily calculated by the formula:

The hydrated salt  is stable at room temperature and has a different color than the dehydrated version.  The hydrated crystals are blue, while the dehydrated crystals are white.  This makes it easy to see when the crystals are completely dehydrated.