Molar MassMolar mass of hydrogen (H2)
The molar mass of hydrogen, H2, is 2.02 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 2.02 grams.
2.02 g/mol
2 × 1.0080 = 2.016 · 2 atoms in the formula · also called dihydrogen
About hydrogen
Hydrogen is the lightest of all gases. Its molecule has two atoms, giving a molar mass of only about 2 grams.
Other names: dihydrogen.
The working
The formula H2 has 2 hydrogen atoms. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 100.00% |
2 × 1.0080 = 2.016 g/mol
With the rounded atomic masses used in many school books (H = 1) the answer is 2 g/mol. The difference from 2.02 is small, but it shows why you should use the values your question paper gives.
Percentage composition
H2 contains only hydrogen, so hydrogen makes up 100% of its mass. Each molecule has 2 atoms, which is why the molar mass is 2 times the atomic weight 1.0080.
Worked examples
Grams to moles. How many moles are in 25 g of H2? Divide the mass by the molar mass: 25 ÷ 2.016 = 12.4 mol.
Moles to grams. What is the mass of 0.1 mol of H2? Multiply: 0.1 × 2.016 = 0.2016 g.
Counting particles. 25 g contains 12.4 mol × 6.022 × 1023 = 7.468 × 1024 molecules, and 1.494 × 1025 atoms in all.
As a gas. At 0 °C and 1 atmosphere one mole of an ideal gas occupies 22.4 litres, so the density of H2 is about 2.016 ÷ 22.4 = 0.09 g per litre. Air averages about 29 g/mol, so this gas is lighter than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 0.2016 g |
| 0.5 mol | 1.008 g |
| 1 mol | 2.016 g |
| 2 mol | 4.032 g |
| 5 mol | 10.08 g |
| Mass | Moles |
|---|---|
| 1 g | 0.496 mol |
| 10 g | 4.96 mol |
| 100 g | 49.6 mol |
| 1 kg | 496 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of H2?
The molar mass of hydrogen (H2) is 2.02 g/mol, or 2.016 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of H2?
100 ÷ 2.016 = 49.6 mol.
What is the mass of one molecule of H2?
Divide the molar mass by the Avogadro constant: 2.016 ÷ 6.022 × 1023 = 3.348 × 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.