Molar MassHydrates and water of crystallisation
Many salts form crystals that contain a fixed amount of water. This water is part of the crystal, not dampness. It is called water of crystallisation, and the salt is called a hydrate. The salt without its water is called anhydrous.
Reading the formula
Blue copper sulphate is written CuSO4·5H2O. Read it as "copper sulphate, five water". The dot is not a multiplication sign. The 5 in front of H2O multiplies the whole water molecule: 10 hydrogen atoms and 5 oxygen atoms, in addition to the 4 oxygen atoms of the sulphate.
The number of waters is shown in the name by a Greek prefix: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6, hepta 7, octa 8, deca 10, dodeca 12.
Molar mass of a hydrate
Add the molar mass of the anhydrous salt and of all the water.
| Hydrate | Anhydrous salt | Water | Molar mass, g/mol |
|---|---|---|---|
| CuSO4·5H2O | 159.60 | 5 × 18.015 | 249.68 |
| Na2CO3·10H2O | 105.99 | 10 × 18.015 | 286.14 |
| MgSO4·7H2O | 120.36 | 7 × 18.015 | 246.47 |
| FeSO4·7H2O | 151.90 | 7 × 18.015 | 278.01 |
Why it matters when you weigh
If a method asks for 0.1 mol of copper sulphate and the bottle holds the blue crystals, you must weigh 0.1 × 249.68 = 24.97 g, not 15.96 g. Using the anhydrous figure would leave you about a third short. Always read the label on the bottle and use the formula printed there.
Percentage of water
In CuSO4·5H2O the water has a mass of 90.08 out of 249.68, so 36.1% of the crystal is water. In Epsom salt, MgSO4·7H2O, it is 51.2%: more than half.
Finding the number of waters by heating
Gentle heating drives off the water and leaves the anhydrous salt. Weigh before and after.
Example. 2.50 g of blue copper sulphate crystals leave 1.60 g of white powder.
- Water lost: 2.50 − 1.60 = 0.90 g.
- Moles of anhydrous CuSO4: 1.60 ÷ 159.60 = 0.0100 mol.
- Moles of water: 0.90 ÷ 18.02 = 0.0500 mol.
- Ratio of water to salt: 0.0500 ÷ 0.0100 = 4.98, which is 5 to the nearest whole number.
So the formula is CuSO4·5H2O. The result in a real class is often a little under 5, because some water stays behind, or a little over, because strong heating starts to break down the salt itself.
Efflorescence and deliquescence
Some hydrates lose water to dry air and turn powdery; washing soda does this. It is called efflorescence. Other substances, such as calcium chloride, take up so much water from the air that they dissolve in it, which is deliquescence. Either change alters the mass per mole of what is in the bottle, one reason chemists choose stable compounds as standards.
All the hydrates on this site are listed on the hydrates page. To type one into the calculator, use a full stop for the dot: CuSO4.5H2O.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.