Analysis of an Unknown Mixture by Dehydration

PURPOSE OF EXPERIMENT Study some properties of hydrates, and determine the percent composition by mass of an unknown mixture of a hydrated salt and an anhydrous salt. 

Some compounds that do not contain either adsorbed or hydrate water still give off water on heating as a result of decomposition of the compounds. This decomposition frequently involves the splitting out of water between two molecules of the compound in what are called condensation reactions. There is at least one example, shown below, that illustrates a loss of hydrate water followed by three condensation reactions. 

For those hydrates that don t decompose thermally, the extent of hydration can be determined by weighing the solid sample before and after heating. The mass after heating is for the anhydrous salt, and the difference in the two masses gives the mass of water originally present. Moles of both the anhydrous salt and water can then be calculated, and the mole ratio of water to anhydrous salt can be determined. These ideas are illustrated with a hydrate of barium chloride, BaCl2 xH20. 

and Purcell, K. F., Chemistry and Chemical Reactivity. Saunders College Publishing, Philadelphia, 1987, sections 2.5, 2.11c, 2.12, and 4.1. 

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