Molar MassMolar mass of copper(II) sulphate pentahydrate (CuSO4·5H2O)
The molar mass of copper(II) sulphate pentahydrate, CuSO4·5H2O, is 249.68 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 249.68 grams.
249.68 g/mol
63.546 + 32.06 + 9 × 15.999 + 10 × 1.0080 = 249.677 · 21 atoms in the formula · also called blue vitriol
About copper(II) sulphate pentahydrate
Blue vitriol is the familiar blue crystal of the school laboratory. Each formula unit holds five water molecules. On heating it loses them and turns white, and the loss in mass is used to find the number 5 by experiment.
Other names: blue vitriol.
The working
The formula CuSO4·5H2O has 1 copper atom, 1 sulphur atom, 9 oxygen atoms and 10 hydrogen atoms, 21 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Copper (Cu) | 1 | 63.546 | 63.546 | 25.45% |
| Sulphur (S) | 1 | 32.06 | 32.060 | 12.84% |
| Oxygen (O) | 9 | 15.999 | 143.991 | 57.67% |
| Hydrogen (H) | 10 | 1.0080 | 10.080 | 4.04% |
63.546 + 32.06 + 9 × 15.999 + 10 × 1.0080 = 249.677 g/mol
With the rounded atomic masses used in many school books (Cu = 63.5, S = 32, O = 16, H = 1) the answer is 249.5 g/mol. The difference from 249.68 is too small to matter in most school sums.
The water in the crystal
The part after the dot is water of crystallisation: 5 molecules of water for each CuSO4. That water has a mass of 5 × 18.015 = 90.08 g/mol, which is 36.1% of the crystal. Without it, the anhydrous part CuSO4 has a molar mass of 159.60 g/mol. Heating drives the water off, so a sample loses up to 36.1% of its mass. How this is used to find the number of waters is shown on the hydrates page.
Percentage composition
By mass, CuSO4·5H2O is 25.45% copper, 12.84% sulphur, 57.67% oxygen and 4.04% hydrogen. The largest share comes from oxygen. In a 100 g sample there are 25.5 g of copper, 12.8 g of sulphur, 57.7 g of oxygen and 4.0 g of hydrogen.
Notice that hydrogen has more atoms in the formula (10) than oxygen (9), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.
Worked examples
Grams to moles. How many moles are in 50 g of CuSO4·5H2O? Divide the mass by the molar mass: 50 ÷ 249.68 = 0.2003 mol.
Moles to grams. What is the mass of 0.4 mol of CuSO4·5H2O? Multiply: 0.4 × 249.68 = 99.87 g.
Counting particles. 50 g contains 0.2003 mol × 6.022 × 1023 = 1.206 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 249.68 = 24.97 g of CuSO4·5H2O in each litre. For 250 mL, weigh 6.242 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 24.97 g |
| 0.5 mol | 124.8 g |
| 1 mol | 249.7 g |
| 2 mol | 499.4 g |
| 5 mol | 1248 g |
| Mass | Moles |
|---|---|
| 1 g | 0.004005 mol |
| 10 g | 0.04005 mol |
| 100 g | 0.4005 mol |
| 1 kg | 4.005 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of CuSO4.5H2O?
The molar mass of copper(II) sulphate pentahydrate (CuSO4.5H2O) is 249.68 g/mol, or 249.677 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of CuSO4.5H2O?
100 ÷ 249.68 = 0.4005 mol.
What is the mass of one formula unit of CuSO4.5H2O?
Divide the molar mass by the Avogadro constant: 249.68 ÷ 6.022 × 1023 = 4.146 × 10-22 g.
What is the percentage of oxygen in CuSO4.5H2O?
Oxygen makes up 57.67% of the mass of CuSO4.5H2O: 9 × 15.999 ÷ 249.677 × 100.
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.