Molar MassMolar mass of aluminium sulphate (Al2(SO4)3)
The molar mass of aluminium sulphate, Al2(SO4)3, is 342.13 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 342.13 grams.
342.13 g/mol
2 × 26.982 + 3 × 32.06 + 12 × 15.999 = 342.132 · 17 atoms in the formula · also called aluminum sulfate
About aluminium sulphate
Aluminium sulphate is used to clear muddy water and in paper making. Its formula needs care: two aluminium atoms, and a bracket of three sulphate groups giving three sulphur and twelve oxygen atoms.
Other names: aluminum sulfate.
The working
The formula Al2(SO4)3 has 2 aluminium atoms, 3 sulphur atoms and 12 oxygen atoms, 17 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Aluminium (Al) | 2 | 26.982 | 53.964 | 15.77% |
| Sulphur (S) | 3 | 32.06 | 96.180 | 28.11% |
| Oxygen (O) | 12 | 15.999 | 191.988 | 56.12% |
2 × 26.982 + 3 × 32.06 + 12 × 15.999 = 342.132 g/mol
With the rounded atomic masses used in many school books (Al = 27, S = 32, O = 16) the answer is 342 g/mol. The difference from 342.13 is too small to matter in most school sums.
Percentage composition
By mass, Al2(SO4)3 is 15.77% aluminium, 28.11% sulphur and 56.12% oxygen. The largest share comes from oxygen. In a 100 g sample there are 15.8 g of aluminium, 28.1 g of sulphur and 56.1 g of oxygen.
Worked examples
Grams to moles. How many moles are in 20 g of Al2(SO4)3? Divide the mass by the molar mass: 20 ÷ 342.13 = 0.05846 mol.
Moles to grams. What is the mass of 0.5 mol of Al2(SO4)3? Multiply: 0.5 × 342.13 = 171.1 g.
Counting particles. 20 g contains 0.05846 mol × 6.022 × 1023 = 3.520 × 1022 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 342.13 = 34.21 g of Al2(SO4)3 in each litre. For 250 mL, weigh 8.553 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 34.21 g |
| 0.5 mol | 171.1 g |
| 1 mol | 342.1 g |
| 2 mol | 684.3 g |
| 5 mol | 1711 g |
| Mass | Moles |
|---|---|
| 1 g | 0.002923 mol |
| 10 g | 0.02923 mol |
| 100 g | 0.2923 mol |
| 1 kg | 2.923 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Al2(SO4)3?
The molar mass of aluminium sulphate (Al2(SO4)3) is 342.13 g/mol, or 342.132 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Al2(SO4)3?
100 ÷ 342.13 = 0.2923 mol.
What is the mass of one formula unit of Al2(SO4)3?
Divide the molar mass by the Avogadro constant: 342.13 ÷ 6.022 × 1023 = 5.681 × 10-22 g.
What is the percentage of oxygen in Al2(SO4)3?
Oxygen makes up 56.12% of the mass of Al2(SO4)3: 12 × 15.999 ÷ 342.132 × 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.