Molar MassMolar mass of bromine (Br2)
The molar mass of bromine, Br2, is 159.81 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 159.81 grams.
About bromine
Bromine is a dark red-brown liquid at room temperature, one of only two liquid elements, and gives off a brown vapour. It is listed here with the other two-atom elements.
The working
The formula Br2 has 2 bromine atoms. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Bromine (Br) | 2 | 79.904 | 159.808 | 100.00% |
2 × 79.904 = 159.808 g/mol
With the rounded atomic masses used in many school books (Br = 80) the answer is 160 g/mol. The difference from 159.81 is too small to matter in most school sums.
Percentage composition
Br2 contains only bromine, so bromine makes up 100% of its mass. Each molecule has 2 atoms, which is why the molar mass is 2 times the atomic weight 79.904.
Worked examples
Grams to moles. How many moles are in 40 g of Br2? Divide the mass by the molar mass: 40 ÷ 159.81 = 0.2503 mol.
Moles to grams. What is the mass of 3 mol of Br2? Multiply: 3 × 159.81 = 479.4 g.
Counting particles. 40 g contains 0.2503 mol × 6.022 × 1023 = 1.507 × 1023 molecules, and 3.015 × 1023 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 15.98 g |
| 0.5 mol | 79.9 g |
| 1 mol | 159.8 g |
| 2 mol | 319.6 g |
| 5 mol | 799 g |
| Mass | Moles |
|---|---|
| 1 g | 0.006258 mol |
| 10 g | 0.06258 mol |
| 100 g | 0.6258 mol |
| 1 kg | 6.258 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Br2?
The molar mass of bromine (Br2) is 159.81 g/mol, or 159.808 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Br2?
100 ÷ 159.81 = 0.6258 mol.
What is the mass of one molecule of Br2?
Divide the molar mass by the Avogadro constant: 159.81 ÷ 6.022 × 1023 = 2.654 × 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.