Molar MassMolar mass of chlorine (Cl2)
The molar mass of chlorine, Cl2, is 70.90 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 70.90 grams.
About chlorine
Chlorine is a greenish yellow poisonous gas, about two and a half times as heavy as air, used to disinfect water.
The working
The formula Cl2 has 2 chlorine atoms. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Chlorine (Cl) | 2 | 35.45 | 70.900 | 100.00% |
2 × 35.45 = 70.900 g/mol
With the rounded atomic masses used in many school books (Cl = 35.5) the answer is 71 g/mol. The difference from 70.90 is too small to matter in most school sums.
Percentage composition
Cl2 contains only chlorine, so chlorine makes up 100% of its mass. Each molecule has 2 atoms, which is why the molar mass is 2 times the atomic weight 35.45.
Worked examples
Grams to moles. How many moles are in 50 g of Cl2? Divide the mass by the molar mass: 50 ÷ 70.90 = 0.7052 mol.
Moles to grams. What is the mass of 0.4 mol of Cl2? Multiply: 0.4 × 70.90 = 28.36 g.
Counting particles. 50 g contains 0.7052 mol × 6.022 × 1023 = 4.247 × 1023 molecules, and 8.494 × 1023 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 Cl2 is about 70.90 ÷ 22.4 = 3.16 g per litre. Air averages about 29 g/mol, so this gas is heavier than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 7.09 g |
| 0.5 mol | 35.45 g |
| 1 mol | 70.9 g |
| 2 mol | 141.8 g |
| 5 mol | 354.5 g |
| Mass | Moles |
|---|---|
| 1 g | 0.0141 mol |
| 10 g | 0.141 mol |
| 100 g | 1.41 mol |
| 1 kg | 14.1 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Cl2?
The molar mass of chlorine (Cl2) is 70.90 g/mol, or 70.900 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Cl2?
100 ÷ 70.90 = 1.41 mol.
What is the mass of one molecule of Cl2?
Divide the molar mass by the Avogadro constant: 70.90 ÷ 6.022 × 1023 = 1.177 × 10-22 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.