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Molar mass of iron(II) sulphate heptahydrate (FeSO4·7H2O)

The molar mass of iron(II) sulphate heptahydrate, FeSO4·7H2O, is 278.01 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 278.01 grams.

278.01 g/mol

55.845 + 32.06 + 11 × 15.999 + 14 × 1.0080 = 278.006 · 27 atoms in the formula · also called green vitriol

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About iron(II) sulphate heptahydrate

Green vitriol forms pale green crystals. It is the form of iron(II) sulphate actually weighed out in the laboratory.

Other names: green vitriol.

The working

The formula FeSO4·7H2O has 1 iron atom, 1 sulphur atom, 11 oxygen atoms and 14 hydrogen atoms, 27 atoms in all. Multiply each count by the atomic weight of the element and add:

ElementAtomsAtomic weightTotalShare by mass
Iron (Fe)155.84555.84520.09%
Sulphur (S)132.0632.06011.53%
Oxygen (O)1115.999175.98963.30%
Hydrogen (H)141.008014.1125.08%

55.845 + 32.06 + 11 × 15.999 + 14 × 1.0080 = 278.006 g/mol

With the rounded atomic masses used in many school books (Fe = 56, S = 32, O = 16, H = 1) the answer is 278 g/mol. The difference from 278.01 is too small to matter in most school sums.

The water in the crystal

The part after the dot is water of crystallisation: 7 molecules of water for each FeSO4. That water has a mass of 7 × 18.015 = 126.11 g/mol, which is 45.4% of the crystal. Without it, the anhydrous part FeSO4 has a molar mass of 151.90 g/mol. Heating drives the water off, so a sample loses up to 45.4% of its mass. How this is used to find the number of waters is shown on the hydrates page.

Percentage composition

By mass, FeSO4·7H2O is 20.09% iron, 11.53% sulphur, 63.30% oxygen and 5.08% hydrogen. The largest share comes from oxygen. In a 100 g sample there are 20.1 g of iron, 11.5 g of sulphur, 63.3 g of oxygen and 5.1 g of hydrogen.

Notice that hydrogen has more atoms in the formula (14) than oxygen (11), 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 20 g of FeSO4·7H2O? Divide the mass by the molar mass: 20 ÷ 278.01 = 0.07194 mol.

Moles to grams. What is the mass of 0.5 mol of FeSO4·7H2O? Multiply: 0.5 × 278.01 = 139 g.

Counting particles. 20 g contains 0.07194 mol × 6.022 × 1023 = 4.332 × 1022 formula units.

Making a solution. A 0.1 mol/L solution contains 0.1 × 278.01 = 27.8 g of FeSO4·7H2O in each litre. For 250 mL, weigh 6.95 g and make it up to the mark with water.

Quick conversion tables

AmountMass
0.1 mol27.8 g
0.5 mol139 g
1 mol278 g
2 mol556 g
5 mol1390 g
MassMoles
1 g0.003597 mol
10 g0.03597 mol
100 g0.3597 mol
1 kg3.597 mol

For other amounts use the grams and moles converter.

Questions people ask

What is the molar mass of FeSO4.7H2O?

The molar mass of iron(II) sulphate heptahydrate (FeSO4.7H2O) is 278.01 g/mol, or 278.006 g/mol before rounding, using IUPAC abridged atomic weights.

How many moles are in 100 g of FeSO4.7H2O?

100 ÷ 278.01 = 0.3597 mol.

What is the mass of one formula unit of FeSO4.7H2O?

Divide the molar mass by the Avogadro constant: 278.01 ÷ 6.022 × 1023 = 4.616 × 10-22 g.

What is the percentage of oxygen in FeSO4.7H2O?

Oxygen makes up 63.30% of the mass of FeSO4.7H2O: 11 × 15.999 ÷ 278.006 × 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.

More hydrates

All hydrates · All compounds