Molar MassMolar mass of iron(II) sulphate (FeSO4)
The molar mass of iron(II) sulphate, FeSO4, is 151.90 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 151.90 grams.
151.90 g/mol
55.845 + 32.06 + 4 × 15.999 = 151.901 · 6 atoms in the formula · also called ferrous sulfate
About iron(II) sulphate
Iron(II) sulphate is used in iron tablets and in redox titrations. The pale green crystals usually met in the laboratory are the heptahydrate, so check which form is meant before using this figure.
Other names: ferrous sulfate.
The working
The formula FeSO4 has 1 iron atom, 1 sulphur atom and 4 oxygen atoms, 6 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Iron (Fe) | 1 | 55.845 | 55.845 | 36.76% |
| Sulphur (S) | 1 | 32.06 | 32.060 | 21.11% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 42.13% |
55.845 + 32.06 + 4 × 15.999 = 151.901 g/mol
With the rounded atomic masses used in many school books (Fe = 56, S = 32, O = 16) the answer is 152 g/mol. The difference from 151.90 is too small to matter in most school sums.
Percentage composition
By mass, FeSO4 is 36.76% iron, 21.11% sulphur and 42.13% oxygen. The largest share comes from oxygen. In a 100 g sample there are 36.8 g of iron, 21.1 g of sulphur and 42.1 g of oxygen.
Worked examples
Grams to moles. How many moles are in 25 g of FeSO4? Divide the mass by the molar mass: 25 ÷ 151.90 = 0.1646 mol.
Moles to grams. What is the mass of 0.1 mol of FeSO4? Multiply: 0.1 × 151.90 = 15.19 g.
Counting particles. 25 g contains 0.1646 mol × 6.022 × 1023 = 9.911 × 1022 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 151.90 = 15.19 g of FeSO4 in each litre. For 250 mL, weigh 3.798 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 15.19 g |
| 0.5 mol | 75.95 g |
| 1 mol | 151.9 g |
| 2 mol | 303.8 g |
| 5 mol | 759.5 g |
| Mass | Moles |
|---|---|
| 1 g | 0.006583 mol |
| 10 g | 0.06583 mol |
| 100 g | 0.6583 mol |
| 1 kg | 6.583 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of FeSO4?
The molar mass of iron(II) sulphate (FeSO4) is 151.90 g/mol, or 151.901 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of FeSO4?
100 ÷ 151.90 = 0.6583 mol.
What is the mass of one formula unit of FeSO4?
Divide the molar mass by the Avogadro constant: 151.90 ÷ 6.022 × 1023 = 2.522 × 10-22 g.
What is the percentage of oxygen in FeSO4?
Oxygen makes up 42.13% of the mass of FeSO4: 4 × 15.999 ÷ 151.901 × 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.