Molar MassMolar mass of hydrates
A hydrate is a salt that holds a fixed number of water molecules in its crystals. The water is written after a dot in the formula and its mass is part of the molar mass.
To type a hydrate into the calculator, use a full stop for the dot: CuSO4.5H2O. The number after the dot multiplies the whole water molecule. In titration work the hydrate is what you weigh, so its molar mass, not that of the anhydrous salt, is the one to use.
This page lists 12 hydrates with their formulas and molar masses, lightest first. The lightest here is oxalic acid dihydrate at 126.06 g/mol and the heaviest is potassium aluminium sulphate dodecahydrate at 474.37 g/mol. Tap a name for the full working, the percentage of each element and worked examples.
| Compound | Formula | Molar mass, g/mol |
|---|---|---|
| Oxalic acid dihydrate | H2C2O4·2H2O | 126.06 |
| Calcium chloride dihydrate | CaCl2·2H2O | 147.01 |
| Calcium sulphate dihydrate gypsum | CaSO4·2H2O | 172.16 |
| Barium chloride dihydrate | BaCl2·2H2O | 244.26 |
| Magnesium sulphate heptahydrate Epsom salt | MgSO4·7H2O | 246.47 |
| Sodium thiosulphate pentahydrate hypo | Na2S2O3·5H2O | 248.17 |
| Copper(II) sulphate pentahydrate blue vitriol | CuSO4·5H2O | 249.68 |
| Iron(II) sulphate heptahydrate green vitriol | FeSO4·7H2O | 278.01 |
| Sodium carbonate decahydrate washing soda | Na2CO3·10H2O | 286.14 |
| Zinc sulphate heptahydrate white vitriol | ZnSO4·7H2O | 287.54 |
| Ammonium iron(II) sulphate hexahydrate Mohr's salt | FeSO4·(NH4)2SO4·6H2O | 392.12 |
| Potassium aluminium sulphate dodecahydrate potash alum | KAl(SO4)2·12H2O | 474.37 |
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.
Not in the list?
Type any formula into the calculator. It handles brackets and water of crystallisation and shows the same working as these pages. The method by hand is on the step by step page.
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