Question

How does the law of definite proportions apply to hydrates?

Answer 1

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.