Molar MassMolar mass of hydrogen bromide (HBr)
The molar mass of hydrogen bromide, HBr, is 80.91 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 80.91 grams.
80.91 g/mol
1.0080 + 79.904 = 80.912 · 2 atoms in the formula · also called hydrobromic acid
About hydrogen bromide
Hydrogen bromide is a colourless gas that fumes in moist air. Its solution in water, hydrobromic acid, is a strong acid. Most of the mass of the molecule comes from the single bromine atom.
Other names: hydrobromic acid.
The working
The formula HBr has 1 hydrogen atom and 1 bromine atom, 2 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Hydrogen (H) | 1 | 1.0080 | 1.008 | 1.25% |
| Bromine (Br) | 1 | 79.904 | 79.904 | 98.75% |
1.0080 + 79.904 = 80.912 g/mol
With the rounded atomic masses used in many school books (H = 1, Br = 80) the answer is 81 g/mol. The difference from 80.91 is too small to matter in most school sums.
Percentage composition
By mass, HBr is 1.25% hydrogen and 98.75% bromine. The largest share comes from bromine. In a 100 g sample there are 1.2 g of hydrogen and 98.8 g of bromine.
Worked examples
Grams to moles. How many moles are in 40 g of HBr? Divide the mass by the molar mass: 40 ÷ 80.91 = 0.4944 mol.
Moles to grams. What is the mass of 3 mol of HBr? Multiply: 3 × 80.91 = 242.7 g.
Counting particles. 40 g contains 0.4944 mol × 6.022 × 1023 = 2.977 × 1023 molecules, and 5.954 × 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 HBr is about 80.91 ÷ 22.4 = 3.61 g per litre. Air averages about 29 g/mol, so this gas is heavier than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 8.091 g |
| 0.5 mol | 40.46 g |
| 1 mol | 80.91 g |
| 2 mol | 161.8 g |
| 5 mol | 404.6 g |
| Mass | Moles |
|---|---|
| 1 g | 0.01236 mol |
| 10 g | 0.1236 mol |
| 100 g | 1.236 mol |
| 1 kg | 12.36 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of HBr?
The molar mass of hydrogen bromide (HBr) is 80.91 g/mol, or 80.912 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of HBr?
100 ÷ 80.91 = 1.236 mol.
What is the mass of one molecule of HBr?
Divide the molar mass by the Avogadro constant: 80.91 ÷ 6.022 × 1023 = 1.344 × 10-22 g.
What is the percentage of bromine in HBr?
Bromine makes up 98.75% of the mass of HBr: 1 × 79.904 ÷ 80.912 × 100.
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.