Molar MassMolar mass of sodium carbonate decahydrate (Na2CO3·10H2O)
The molar mass of sodium carbonate decahydrate, Na2CO3·10H2O, is 286.14 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 286.14 grams.
286.14 g/mol
2 × 22.990 + 12.011 + 13 × 15.999 + 20 × 1.0080 = 286.138 · 36 atoms in the formula · also called washing soda
About sodium carbonate decahydrate
Washing soda holds ten water molecules for each sodium carbonate, so water is well over half its mass. Left in dry air the crystals lose water and turn powdery.
Other names: washing soda.
The working
The formula Na2CO3·10H2O has 2 sodium atoms, 1 carbon atom, 13 oxygen atoms and 20 hydrogen atoms, 36 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 | 16.07% |
| Carbon (C) | 1 | 12.011 | 12.011 | 4.20% |
| Oxygen (O) | 13 | 15.999 | 207.987 | 72.69% |
| Hydrogen (H) | 20 | 1.0080 | 20.160 | 7.05% |
2 × 22.990 + 12.011 + 13 × 15.999 + 20 × 1.0080 = 286.138 g/mol
With the rounded atomic masses used in many school books (Na = 23, C = 12, O = 16, H = 1) the answer is 286 g/mol. The difference from 286.14 is too small to matter in most school sums.
The water in the crystal
The part after the dot is water of crystallisation: 10 molecules of water for each Na2CO3. That water has a mass of 10 × 18.015 = 180.15 g/mol, which is 63.0% of the crystal. Without it, the anhydrous part Na2CO3 has a molar mass of 105.99 g/mol. Heating drives the water off, so a sample loses up to 63.0% of its mass. How this is used to find the number of waters is shown on the hydrates page.
Percentage composition
By mass, Na2CO3·10H2O is 16.07% sodium, 4.20% carbon, 72.69% oxygen and 7.05% hydrogen. The largest share comes from oxygen. In a 100 g sample there are 16.1 g of sodium, 4.2 g of carbon, 72.7 g of oxygen and 7.0 g of hydrogen.
Worked examples
Grams to moles. How many moles are in 100 g of Na2CO3·10H2O? Divide the mass by the molar mass: 100 ÷ 286.14 = 0.3495 mol.
Moles to grams. What is the mass of 2 mol of Na2CO3·10H2O? Multiply: 2 × 286.14 = 572.3 g.
Counting particles. 100 g contains 0.3495 mol × 6.022 × 1023 = 2.105 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 286.14 = 28.61 g of Na2CO3·10H2O in each litre. For 250 mL, weigh 7.153 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 28.61 g |
| 0.5 mol | 143.1 g |
| 1 mol | 286.1 g |
| 2 mol | 572.3 g |
| 5 mol | 1431 g |
| Mass | Moles |
|---|---|
| 1 g | 0.003495 mol |
| 10 g | 0.03495 mol |
| 100 g | 0.3495 mol |
| 1 kg | 3.495 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Na2CO3.10H2O?
The molar mass of sodium carbonate decahydrate (Na2CO3.10H2O) is 286.14 g/mol, or 286.138 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Na2CO3.10H2O?
100 ÷ 286.14 = 0.3495 mol.
What is the mass of one formula unit of Na2CO3.10H2O?
Divide the molar mass by the Avogadro constant: 286.14 ÷ 6.022 × 1023 = 4.751 × 10-22 g.
What is the percentage of oxygen in Na2CO3.10H2O?
Oxygen makes up 72.69% of the mass of Na2CO3.10H2O: 13 × 15.999 ÷ 286.138 × 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.