Molar MassMolar mass of carbon monoxide (CO)
The molar mass of carbon monoxide, CO, is 28.01 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 28.01 grams.
About carbon monoxide
Carbon monoxide is a colourless, odourless and very poisonous gas formed when fuels burn in too little air. Mind the capital letters: CO is carbon monoxide, but Co is the metal cobalt.
The working
The formula CO has 1 carbon atom and 1 oxygen 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 |
|---|---|---|---|---|
| Carbon (C) | 1 | 12.011 | 12.011 | 42.88% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 57.12% |
12.011 + 15.999 = 28.010 g/mol
With the rounded atomic masses used in many school books (C = 12, O = 16) the answer is 28 g/mol. The difference from 28.01 is too small to matter in most school sums.
Percentage composition
By mass, CO is 42.88% carbon and 57.12% oxygen. The largest share comes from oxygen. In a 100 g sample there are 42.9 g of carbon and 57.1 g of oxygen.
Worked examples
Grams to moles. How many moles are in 100 g of CO? Divide the mass by the molar mass: 100 ÷ 28.01 = 3.57 mol.
Moles to grams. What is the mass of 2 mol of CO? Multiply: 2 × 28.01 = 56.02 g.
Counting particles. 100 g contains 3.57 mol × 6.022 × 1023 = 2.150 × 1024 molecules, and 4.300 × 1024 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 CO is about 28.01 ÷ 22.4 = 1.25 g per litre. Air averages about 29 g/mol, so this gas is lighter than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 2.801 g |
| 0.5 mol | 14 g |
| 1 mol | 28.01 g |
| 2 mol | 56.02 g |
| 5 mol | 140 g |
| Mass | Moles |
|---|---|
| 1 g | 0.0357 mol |
| 10 g | 0.357 mol |
| 100 g | 3.57 mol |
| 1 kg | 35.7 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of CO?
The molar mass of carbon monoxide (CO) is 28.01 g/mol, or 28.010 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of CO?
100 ÷ 28.01 = 3.57 mol.
What is the mass of one molecule of CO?
Divide the molar mass by the Avogadro constant: 28.01 ÷ 6.022 × 1023 = 4.651 × 10-23 g.
What is the percentage of oxygen in CO?
Oxygen makes up 57.12% of the mass of CO: 1 × 15.999 ÷ 28.010 × 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.