Molar MassMolar mass of helium (He)
The molar mass of helium, He, is 4.00 g/mol. One mole of it, which is 6.022 × 1023 atoms, has a mass of 4.00 grams.
About helium
Helium is a noble gas that exists as single atoms, so its molar mass equals its atomic weight. It is far lighter than air and is used in balloons.
The working
The formula He has 1 helium atom. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Helium (He) | 1 | 4.0026 | 4.003 | 100.00% |
4.0026 = 4.003 g/mol
With the rounded atomic masses used in many school books (He = 4) the answer is 4 g/mol. The difference from 4.00 is too small to matter in most school sums.
Percentage composition
He contains only helium, so helium 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 250 g of He? Divide the mass by the molar mass: 250 ÷ 4.003 = 62.46 mol.
Moles to grams. What is the mass of 0.2 mol of He? Multiply: 0.2 × 4.003 = 0.8005 g.
Counting particles. 250 g contains 62.46 mol × 6.022 × 1023 = 3.761 × 1025 atoms.
As a gas. At 0 °C and 1 atmosphere one mole of an ideal gas occupies 22.4 litres, so the density of He is about 4.003 ÷ 22.4 = 0.18 g per litre. Air averages about 29 g/mol, so this gas is lighter than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 0.4003 g |
| 0.5 mol | 2.001 g |
| 1 mol | 4.003 g |
| 2 mol | 8.005 g |
| 5 mol | 20.01 g |
| Mass | Moles |
|---|---|
| 1 g | 0.2498 mol |
| 10 g | 2.498 mol |
| 100 g | 24.98 mol |
| 1 kg | 249.8 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of He?
The molar mass of helium (He) is 4.00 g/mol, or 4.003 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of He?
100 ÷ 4.003 = 24.98 mol.
What is the mass of one atom of He?
Divide the molar mass by the Avogadro constant: 4.003 ÷ 6.022 × 1023 = 6.646 × 10-24 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.