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.