How does the law of definite proportions apply to hydrates?

The law of definite proportions would state that a hydrate always contain exactly the same proportion of salt and water by mass.
strictly speaking, the law of definite proportion 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 compound.
Hydrates are salt that have a certain amount of  water asa part of their structure.
The water is chemically combined with the compound in a definite ratio.

Ifound this on google “laws of chemical combination. it states that matter can neither be created nor destroyed. it stated that a given compound always contains exactly the same proportion of elements by weight.” i hope this you for what your looking for.