Molar MassMolar mass of potassium bromide (KBr)
The molar mass of potassium bromide, KBr, is 119.00 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 119.00 grams.
About potassium bromide
Potassium bromide is a white salt once used widely in photography. Pressed into clear discs, it holds samples for infrared spectroscopy because it does not absorb infrared light.
The working
The formula KBr has 1 potassium 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 |
|---|---|---|---|---|
| Potassium (K) | 1 | 39.098 | 39.098 | 32.85% |
| Bromine (Br) | 1 | 79.904 | 79.904 | 67.15% |
39.098 + 79.904 = 119.002 g/mol
With the rounded atomic masses used in many school books (K = 39, Br = 80) the answer is 119 g/mol. The difference from 119.00 is too small to matter in most school sums.
Percentage composition
By mass, KBr is 32.85% potassium and 67.15% bromine. The largest share comes from bromine. In a 100 g sample there are 32.9 g of potassium and 67.1 g of bromine.
Worked examples
Grams to moles. How many moles are in 100 g of KBr? Divide the mass by the molar mass: 100 ÷ 119.00 = 0.8403 mol.
Moles to grams. What is the mass of 2 mol of KBr? Multiply: 2 × 119.00 = 238 g.
Counting particles. 100 g contains 0.8403 mol × 6.022 × 1023 = 5.061 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 119.00 = 11.9 g of KBr in each litre. For 250 mL, weigh 2.975 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 11.9 g |
| 0.5 mol | 59.5 g |
| 1 mol | 119 g |
| 2 mol | 238 g |
| 5 mol | 595 g |
| Mass | Moles |
|---|---|
| 1 g | 0.008403 mol |
| 10 g | 0.08403 mol |
| 100 g | 0.8403 mol |
| 1 kg | 8.403 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of KBr?
The molar mass of potassium bromide (KBr) is 119.00 g/mol, or 119.002 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of KBr?
100 ÷ 119.00 = 0.8403 mol.
What is the mass of one formula unit of KBr?
Divide the molar mass by the Avogadro constant: 119.00 ÷ 6.022 × 1023 = 1.976 × 10-22 g.
What is the percentage of bromine in KBr?
Bromine makes up 67.15% of the mass of KBr: 1 × 79.904 ÷ 119.002 × 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.