Molar MassMolar mass of potassium carbonate (K2CO3)
The molar mass of potassium carbonate, K2CO3, is 138.20 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 138.20 grams.
138.20 g/mol
2 × 39.098 + 12.011 + 3 × 15.999 = 138.204 · 6 atoms in the formula · also called potash, pearl ash
About potassium carbonate
Potassium carbonate was first obtained from wood ash, which gave potassium its name. It is used in making soap and hard glass.
Other names: potash, pearl ash.
The working
The formula K2CO3 has 2 potassium atoms, 1 carbon atom and 3 oxygen atoms, 6 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) | 2 | 39.098 | 78.196 | 56.58% |
| Carbon (C) | 1 | 12.011 | 12.011 | 8.69% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 34.73% |
2 × 39.098 + 12.011 + 3 × 15.999 = 138.204 g/mol
With the rounded atomic masses used in many school books (K = 39, C = 12, O = 16) the answer is 138 g/mol. The difference from 138.20 is too small to matter in most school sums.
Percentage composition
By mass, K2CO3 is 56.58% potassium, 8.69% carbon and 34.73% oxygen. The largest share comes from potassium. In a 100 g sample there are 56.6 g of potassium, 8.7 g of carbon and 34.7 g of oxygen.
Worked examples
Grams to moles. How many moles are in 100 g of K2CO3? Divide the mass by the molar mass: 100 ÷ 138.20 = 0.7236 mol.
Moles to grams. What is the mass of 2 mol of K2CO3? Multiply: 2 × 138.20 = 276.4 g.
Counting particles. 100 g contains 0.7236 mol × 6.022 × 1023 = 4.357 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 138.20 = 13.82 g of K2CO3 in each litre. For 250 mL, weigh 3.455 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 13.82 g |
| 0.5 mol | 69.1 g |
| 1 mol | 138.2 g |
| 2 mol | 276.4 g |
| 5 mol | 691 g |
| Mass | Moles |
|---|---|
| 1 g | 0.007236 mol |
| 10 g | 0.07236 mol |
| 100 g | 0.7236 mol |
| 1 kg | 7.236 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of K2CO3?
The molar mass of potassium carbonate (K2CO3) is 138.20 g/mol, or 138.204 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of K2CO3?
100 ÷ 138.20 = 0.7236 mol.
What is the mass of one formula unit of K2CO3?
Divide the molar mass by the Avogadro constant: 138.20 ÷ 6.022 × 1023 = 2.295 × 10-22 g.
What is the percentage of potassium in K2CO3?
Potassium makes up 56.58% of the mass of K2CO3: 2 × 39.098 ÷ 138.204 × 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.