How does the law of definite proportions apply to hydrates?

1 Answer
Aug 17, 2014

The Law of Definite Proportions would state that a hydrate always contains exactly the same proportion of salt and water by mass.

Strictly speaking, the Law of Definite Proportions states that a compound always contains exactly the same proportion of elements by mass.

But the Law is often applied to groupings of elements in compounds.

Hydrates are salts that have a certain amount of water as a part of their structure. The water is chemically combined with the compound in a definite ratio.

For example, the formula of magnesium sulfate heptahydrate is MgSO₄·7H₂O.

This hydrate always contains 126.1 g of H₂O for every 120.4 g of MgSO₄. That is, the ratio of H₂O to MgSO₄ is

#(126.1"g H₂O")/(120.4"g MgSO₄") = 1.047/1#


A 1.000 g sample of a hydrate of a salt loses 0.361 g of mass on heating. A 1.250 g sample of the same hydrate loses 0.455 g on heating. Does this experiment illustrate the Law of Definite Proportions?


The loss of mass represents the loss of water.

Sample 1 contained 0.361 g of water and 0.639 g of the salt. The ratio of salt to water is

#(0.639"g salt")/(0.361"g water") = 1.77/1#

Sample 2 contained 0.455 g of water and 0.795 g of the salt. The ratio of salt to water is

#(0.795"g salt")/(0.455"g water") = 1.75/1#

The two ratios agree within experimental error. This experiment illustrates the Law of Definite Proportions.