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.
