Molar MassMolar mass of sulphur trioxide (SO3)
The molar mass of sulphur trioxide, SO3, is 80.06 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 80.06 grams.
80.06 g/mol
32.06 + 3 × 15.999 = 80.057 · 4 atoms in the formula · also called sulfur trioxide
About sulphur trioxide
Sulphur trioxide is made from sulphur dioxide in the contact process and is then turned into sulphuric acid.
Other names: sulfur trioxide.
The working
The formula SO3 has 1 sulphur atom and 3 oxygen atoms, 4 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Sulphur (S) | 1 | 32.06 | 32.060 | 40.05% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 59.95% |
32.06 + 3 × 15.999 = 80.057 g/mol
With the rounded atomic masses used in many school books (S = 32, O = 16) the answer is 80 g/mol. The difference from 80.06 is too small to matter in most school sums.
Percentage composition
By mass, SO3 is 40.05% sulphur and 59.95% oxygen. The largest share comes from oxygen. In a 100 g sample there are 40.0 g of sulphur and 60.0 g of oxygen.
Worked examples
Grams to moles. How many moles are in 20 g of SO3? Divide the mass by the molar mass: 20 ÷ 80.06 = 0.2498 mol.
Moles to grams. What is the mass of 0.5 mol of SO3? Multiply: 0.5 × 80.06 = 40.03 g.
Counting particles. 20 g contains 0.2498 mol × 6.022 × 1023 = 1.504 × 1023 molecules, and 6.018 × 1023 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 8.006 g |
| 0.5 mol | 40.03 g |
| 1 mol | 80.06 g |
| 2 mol | 160.1 g |
| 5 mol | 400.3 g |
| Mass | Moles |
|---|---|
| 1 g | 0.01249 mol |
| 10 g | 0.1249 mol |
| 100 g | 1.249 mol |
| 1 kg | 12.49 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of SO3?
The molar mass of sulphur trioxide (SO3) is 80.06 g/mol, or 80.057 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of SO3?
100 ÷ 80.06 = 1.249 mol.
What is the mass of one molecule of SO3?
Divide the molar mass by the Avogadro constant: 80.06 ÷ 6.022 × 1023 = 1.329 × 10-22 g.
What is the percentage of oxygen in SO3?
Oxygen makes up 59.95% of the mass of SO3: 3 × 15.999 ÷ 80.057 × 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.