Molar MassMolar mass of sodium carbonate (Na2CO3)
The molar mass of sodium carbonate, Na2CO3, is 105.99 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 105.99 grams.
105.99 g/mol
2 × 22.990 + 12.011 + 3 × 15.999 = 105.988 · 6 atoms in the formula · also called soda ash; washing soda when hydrated
About sodium carbonate
Anhydrous sodium carbonate is soda ash, used in making glass, soap and paper. It is a good primary standard for titrating acids because it can be dried and weighed accurately. With ten molecules of water it becomes washing soda.
Other names: soda ash; washing soda when hydrated.
The working
The formula Na2CO3 has 2 sodium 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 |
|---|---|---|---|---|
| Sodium (Na) | 2 | 22.990 | 45.980 | 43.38% |
| Carbon (C) | 1 | 12.011 | 12.011 | 11.33% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 45.29% |
2 × 22.990 + 12.011 + 3 × 15.999 = 105.988 g/mol
With the rounded atomic masses used in many school books (Na = 23, C = 12, O = 16) the answer is 106 g/mol. The difference from 105.99 is too small to matter in most school sums.
Percentage composition
By mass, Na2CO3 is 43.38% sodium, 11.33% carbon and 45.29% oxygen. The largest share comes from oxygen. In a 100 g sample there are 43.4 g of sodium, 11.3 g of carbon and 45.3 g of oxygen.
Worked examples
Grams to moles. How many moles are in 40 g of Na2CO3? Divide the mass by the molar mass: 40 ÷ 105.99 = 0.3774 mol.
Moles to grams. What is the mass of 3 mol of Na2CO3? Multiply: 3 × 105.99 = 318 g.
Counting particles. 40 g contains 0.3774 mol × 6.022 × 1023 = 2.273 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 105.99 = 10.6 g of Na2CO3 in each litre. For 250 mL, weigh 2.65 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 10.6 g |
| 0.5 mol | 52.99 g |
| 1 mol | 106 g |
| 2 mol | 212 g |
| 5 mol | 529.9 g |
| Mass | Moles |
|---|---|
| 1 g | 0.009435 mol |
| 10 g | 0.09435 mol |
| 100 g | 0.9435 mol |
| 1 kg | 9.435 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Na2CO3?
The molar mass of sodium carbonate (Na2CO3) is 105.99 g/mol, or 105.988 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Na2CO3?
100 ÷ 105.99 = 0.9435 mol.
What is the mass of one formula unit of Na2CO3?
Divide the molar mass by the Avogadro constant: 105.99 ÷ 6.022 × 1023 = 1.760 × 10-22 g.
What is the percentage of oxygen in Na2CO3?
Oxygen makes up 45.29% of the mass of Na2CO3: 3 × 15.999 ÷ 105.988 × 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.