Percent Water in Hydrate Calculator
Find the percent water in a hydrate from the mass before and after heating, plus the number of water molecules.
How it works
% water = (mass of hydrate − mass anhydrous) / mass of hydrate × 100 · x = mol H₂O / mol anhydrous
Heating a hydrate drives off the water bound in its crystal, and the mass lost is the mass of that water. Dividing the water mass by the original hydrate mass gives the percent water. If you also know the anhydrous salt's molar mass, converting each mass to moles gives the ratio x — the number of water molecules per formula unit in ·xH₂O.
Worked example
A 24.97 g sample of CuSO₄·xH₂O leaves 15.97 g after heating: water = 9.00 g, so % water = 9.00/24.97 × 100 = 36.0 %. Moles anhydrous = 15.97/159.6 = 0.100; moles water = 9.00/18.015 = 0.500; x = 0.500/0.100 ≈ 5, giving CuSO₄·5H₂O.
Sources: Water of crystallization — Wikipedia
Frequently asked questions
How do I know heating removed all the water?
Heat, cool and weigh repeatedly until the mass stops changing (constant mass). That confirms the sample is fully anhydrous.
Why round x to a whole number?
Waters of crystallization occur in whole-number ratios per formula unit. A result near 5.0 means five waters; small deviations come from measurement error or incomplete drying.
What if I only want the percentage?
Leave the anhydrous molar mass as any positive value — the percent water depends only on the two masses, not the molar mass.
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