The molar mass of sodium hydroxide, NaOH, is 39.996769 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 39.99677 g, and a gram of it is 1.506e+22 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 hydroxide is 22.9898 (Na) + 15.999 (O) + 1.008 (H), which is 39.996769 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Sodium makes up the largest share of the mass: 1 of the 3 atoms in a formula unit is sodium, and they account for 22.9898 g of the 39.99677 g, or 57.48% 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 hydroxide is 39.9968 mg and one micromole is 39.9968 µg, so a balance reading to a milligram places you within 25 µmol. A single formula unit weighs 6.6416e-23 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Caustic soda, used in soap making and drain cleaner. It absorbs water and carbon dioxide from the air, so pellets are weighed quickly and by difference. At 39.99677 g/mol it is the lightest of the 3 bases covered here, and that ranking matters more than it looks: potassium hydroxide has a molar mass of 56.1053 g/mol, so a gram of it contains 28.7% fewer formula units than a gram of sodium hydroxide. 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, 0.79994 g of every 39.99677 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 hydroxide in grams
39.996769- The molar mass of sodium hydroxide 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 hydroxide in grams.
- It is divided by the molar mass, 39.996769 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 hydroxide as a molecule count
0.5 g divided by 39.99677 g/mol is 0.012501 mol, and multiplying by the Avogadro constant gives 7.528e+21 formula units of sodium hydroxide. The same mass is 12.501 mmol, which is the figure a reaction is actually planned in.
5 g of sodium hydroxide as a molecule count
5 g divided by 39.99677 g/mol is 0.12501 mol, and multiplying by the Avogadro constant gives 7.528e+22 formula units of sodium hydroxide. The same mass is 125.01 mmol, which is the figure a reaction is actually planned in.
50 g of sodium hydroxide as a molecule count
50 g divided by 39.99677 g/mol is 1.2501 mol, and multiplying by the Avogadro constant gives 7.528e+23 formula units of sodium hydroxide. The same mass is 1250.1 mmol, which is the figure a reaction is actually planned in.
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Frequently asked questions
What is the molar mass of sodium hydroxide (NaOH)?
It is 39.996769 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 22.9898 (Na) + 15.999 (O) + 1.008 (H). One mole of sodium hydroxide therefore weighs 39.99677 g, and a gram of it is 25.002 mmol.
What percentage of sodium hydroxide is sodium?
57.48% by mass. Each formula unit contains 1 sodium atom contributing 22.9898 g of the 39.99677 g total, so 100 g of sodium hydroxide contains 57.48 g of sodium and a kilogram contains 574.79 g of it.
How many molecules are in one gram of sodium hydroxide?
About 1.506e+22. One gram is 0.025002 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 39.99677.
How much does a single molecule of sodium hydroxide weigh?
6.6416e-23 g, which is 39.996769 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 hydroxide it is 39.996769. Molar mass carries grams per mole: 39.996769 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. Sodium is quoted as 22.98977 rather than a whole number for that reason, and the same applies to the other elements here, which is why sodium hydroxide comes out at 39.996769 g/mol rather than 40.
Molar mass, composition and mole equivalents of sodium hydroxide
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Sodium (Na) | 1 | 22.98977 | 22.98977 | 57.48% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 40% |
| Hydrogen (H) | 1 | 1.008 | 1.008 | 2.52% |
| Total — one mole of sodium hydroxide | 39.996769 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 39.996769 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000040 | 0.0399968 | 6.022e+17 |
| 10 µmol | 0.000399968 | 0.399968 | 6.022e+18 |
| 100 µmol | 0.00399968 | 3.99968 | 6.022e+19 |
| 1 mmol | 0.0399968 | 39.9968 | 6.022e+20 |
| 10 mmol | 0.399968 | 399.968 | 6.022e+21 |
| 50 mmol | 1.99984 | 1999.84 | 3.011e+22 |
| 100 mmol | 3.99968 | 3999.68 | 6.022e+22 |
| 250 mmol | 9.99919 | 9999.19 | 1.506e+23 |
| 500 mmol | 19.9984 | 19,998.4 | 3.011e+23 |
| 750 mmol | 29.9976 | 29,997.6 | 4.517e+23 |
| 1 mol | 39.9968 | 39,996.8 | 6.022e+23 |
| 2 mol | 79.9935 | 79,993.5 | 1.204e+24 |
| 5 mol | 199.984 | 199,984 | 3.011e+24 |
| 10 mol | 399.968 | 399,968 | 6.022e+24 |
Mass is the amount multiplied by 39.996769 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 |
|---|---|---|---|
| Sodium hydroxide (this page) | NaOH | 39.99677 | 25.002 |
| Potassium hydroxide | KOH | 56.1053 | 17.824 |
| Calcium hydroxide | Ca(OH)2 | 74.092 | 13.497 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.