Molar MassMolar mass of argon (Ar)
The molar mass of argon, Ar, is 39.95 g/mol. One mole of it, which is 6.022 × 1023 atoms, has a mass of 39.95 grams.
About argon
Argon is a noble gas made of single atoms and makes up nearly 1 percent of air. It fills light bulbs and shields welding arcs.
The working
The formula Ar has 1 argon atom. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Argon (Ar) | 1 | 39.95 | 39.950 | 100.00% |
39.95 = 39.950 g/mol
With the rounded atomic masses used in many school books (Ar = 40) the answer is 40 g/mol. The difference from 39.95 is too small to matter in most school sums.
Percentage composition
Ar contains only argon, so argon makes up 100% of its mass. It exists as single atoms, so the molar mass equals the atomic weight.
Worked examples
Grams to moles. How many moles are in 10 g of Ar? Divide the mass by the molar mass: 10 ÷ 39.95 = 0.2503 mol.
Moles to grams. What is the mass of 1.5 mol of Ar? Multiply: 1.5 × 39.95 = 59.93 g.
Counting particles. 10 g contains 0.2503 mol × 6.022 × 1023 = 1.507 × 1023 atoms.
As a gas. At 0 °C and 1 atmosphere one mole of an ideal gas occupies 22.4 litres, so the density of Ar is about 39.95 ÷ 22.4 = 1.78 g per litre. Air averages about 29 g/mol, so this gas is heavier than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 3.995 g |
| 0.5 mol | 19.98 g |
| 1 mol | 39.95 g |
| 2 mol | 79.9 g |
| 5 mol | 199.8 g |
| Mass | Moles |
|---|---|
| 1 g | 0.02503 mol |
| 10 g | 0.2503 mol |
| 100 g | 2.503 mol |
| 1 kg | 25.03 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Ar?
The molar mass of argon (Ar) is 39.95 g/mol, or 39.950 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Ar?
100 ÷ 39.95 = 2.503 mol.
What is the mass of one atom of Ar?
Divide the molar mass by the Avogadro constant: 39.95 ÷ 6.022 × 1023 = 6.634 × 10-23 g.
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.