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I assume this question is related to determining the amount of water that is inside the crystalline structure of a hydrate compound such as copper sulfate , magnesium carbonate, calcium sulfate. The water molecules form part of regular crystal structure and a fixed number of water molecules are associated with the hydrate molecules.

Like Calcium Sulfate is actually Ca SO₄ · 2 H₂ O, a dihydrate. So two water molecules for every calcium sulfate are present in the crystal.

Simple experiment:

We take a known mass x of the hydrate and heat it to dryness. When all the water evaporates, the anhydrous compound is left with us. then mass y of the compound is found. Now the mass of water molecule in hydrate is x-y. Its percentage of x gives the percentage of water in the given hydrate.

that is:          (x-y) * 100 / x is the percentage of water.

Ca SO₄ · 2 H₂ O  (solid crystal)  ==>  Ca SO₄ (solid crystal) + 2 H₂ O (vapor)

The ratio

    (y / molecular weight of Ca SO₄) :  [ (x-y) / molecular weight of H₂ O ]

gives the ratio of number of molecules of calcium sulphate to number of molecules of water in the hydrate.

Due to practical errors, we have a small deviation usually less than 1% in the percentages calculated in this manner.

If the experiment is related to find the mass of water socked by raisans he can use W1-W2
 The other methods to find the constration of a solution are-
1- mass/mass %
 2- mass/volume%
 3- volume/volume%
4- no.of moles/volume of solution%