The molar mass of sodium carbonate, Na2CO3, is 105.98754 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 105.9875 g, and a gram of it is 5.682e+21 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
The figure comes from the formula. Sodium carbonate is 2 × 22.9898 (Na) + 12.011 (C) + 3 × 15.999 (O), which is 105.98754 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 3 of the 6 atoms in a formula unit are oxygen, and they account for 47.997 g of the 105.9875 g, or 45.29% by mass. The derivation table below breaks the whole molecule down element by element.
The numbers a bench actually uses sit at the millimole scale. One millimole of sodium carbonate is 105.988 mg and one micromole is 105.988 µg, so a balance reading to a milligram places you within 9.435 µmol. A single formula unit weighs 1.7600e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Washing soda and a glass ingredient. The anhydrous form is the one this figure is for; the decahydrate is 286.14 g per mole. At 105.9875 g/mol it is heavier than 11 of the 34 salts covered here, and that ranking matters more than it looks: potassium nitrate has a molar mass of 101.1023 g/mol, so a gram of it contains 4.83% more formula units than a gram of sodium carbonate. Weigh by mass and you are not weighing equal amounts of substance.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 2.1198 g of every 105.9875 g is not the compound — and any water picked up from the air does the same. Both are systematic, so they shift every solution made from that jar in the same direction.
The formula
m- The mass of sodium carbonate in grams
105.98754- The molar mass of sodium carbonate in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of sodium carbonate in grams.
- It is divided by the molar mass, 105.98754 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of sodium carbonate as a molecule count
0.5 g divided by 105.9875 g/mol is 0.00471754 mol, and multiplying by the Avogadro constant gives 2.841e+21 formula units of sodium carbonate. The same mass is 4.7175 mmol, which is the figure a reaction is actually planned in.
5 g of sodium carbonate as a molecule count
5 g divided by 105.9875 g/mol is 0.0471754 mol, and multiplying by the Avogadro constant gives 2.841e+22 formula units of sodium carbonate. The same mass is 47.175 mmol, which is the figure a reaction is actually planned in.
50 g of sodium carbonate as a molecule count
50 g divided by 105.9875 g/mol is 0.471754 mol, and multiplying by the Avogadro constant gives 2.841e+23 formula units of sodium carbonate. The same mass is 471.75 mmol, which is the figure a reaction is actually planned in.
How to read your score
Frequently asked questions
What is the molar mass of sodium carbonate (Na2CO3)?
It is 105.98754 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 22.9898 (Na) + 12.011 (C) + 3 × 15.999 (O). One mole of sodium carbonate therefore weighs 105.9875 g, and a gram of it is 9.4351 mmol.
What percentage of sodium carbonate is oxygen?
45.29% by mass. Each formula unit contains 3 oxygen atoms contributing 47.997 g of the 105.9875 g total, so 100 g of sodium carbonate contains 45.29 g of oxygen and a kilogram contains 452.86 g of it.
How many molecules are in one gram of sodium carbonate?
About 5.682e+21. One gram is 0.00943507 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 105.9875.
How much does a single molecule of sodium carbonate weigh?
1.7600e-22 g, which is 105.98754 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for sodium carbonate it is 105.98754. Molar mass carries grams per mole: 105.98754 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Oxygen is quoted as 15.999 rather than a whole number for that reason, and the same applies to the other elements here, which is why sodium carbonate comes out at 105.98754 g/mol rather than 106.
Molar mass, composition and mole equivalents of sodium carbonate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 3 | 15.999 | 47.997 | 45.29% |
| Sodium (Na) | 2 | 22.98977 | 45.97954 | 43.38% |
| Carbon (C) | 1 | 12.011 | 12.011 | 11.33% |
| Total — one mole of sodium carbonate | 105.98754 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 105.98754 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000105988 | 0.105988 | 6.022e+17 |
| 10 µmol | 0.00105988 | 1.05988 | 6.022e+18 |
| 100 µmol | 0.0105988 | 10.5988 | 6.022e+19 |
| 1 mmol | 0.105988 | 105.988 | 6.022e+20 |
| 10 mmol | 1.05988 | 1059.88 | 6.022e+21 |
| 50 mmol | 5.29938 | 5299.38 | 3.011e+22 |
| 100 mmol | 10.5988 | 10,598.8 | 6.022e+22 |
| 250 mmol | 26.4969 | 26,496.9 | 1.506e+23 |
| 500 mmol | 52.9938 | 52,993.8 | 3.011e+23 |
| 750 mmol | 79.4907 | 79,490.7 | 4.517e+23 |
| 1 mol | 105.988 | 105,988 | 6.022e+23 |
| 2 mol | 211.975 | 211,975 | 1.204e+24 |
| 5 mol | 529.938 | 529,938 | 3.011e+24 |
| 10 mol | 1059.88 | 1059875.385600 | 6.022e+24 |
Mass is the amount multiplied by 105.98754 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Magnesium chloride | MgCl2 | 95.205 | 10.504 |
| Calcium carbonate | CaCO3 | 100.086 | 9.9914 |
| Potassium nitrate | KNO3 | 101.1023 | 9.891 |
| Sodium carbonate (this page) | Na2CO3 | 105.9875 | 9.4351 |
| Calcium chloride | CaCl2 | 110.978 | 9.0108 |
| Magnesium sulfate | MgSO4 | 120.361 | 8.3083 |
| Ammonium sulfate | (NH4)2SO4 | 132.134 | 7.5681 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.