Going from grams to moles for sodium hydroxide means dividing by its molar mass, 39.996769 g/mol. One gram is 0.025002 mol, or 25.002 mmol; 100 mg is 2.5002 mmol; and a 39.99677 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, NaOH, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.25 mmol of sodium hydroxide; on a three-decimal balance it is 1 mg, or 25 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats weighing.
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.
Purity is the usual gap between the calculation and the balance. A reagent sold at 98% means 0.79994 g of a nominal 39.99677 g is something else, so for exact work you divide the weighed mass by the assay figure on the certificate. For most purposes the difference is smaller than the error in reading the meniscus, but it is systematic rather than random, so it does not average out.
The formula
m- The mass you weighed, in grams
39.996769- The molar mass of sodium hydroxide in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 39.996769 g/mol, the molar mass of sodium hydroxide.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of sodium hydroxide in moles
0.25 ÷ 39.99677 = 0.0062505 mol, or 6.2505 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 25 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of sodium hydroxide in moles
2 ÷ 39.99677 = 0.050004 mol, or 50.004 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 25 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of sodium hydroxide in moles
25 ÷ 39.99677 = 0.62505 mol, or 625.05 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 25 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
How to read your score
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 do I convert grams of sodium hydroxide to moles?
Divide the mass in grams by 39.996769 g/mol. So 1 g is 0.025002 mol, 10 g is 0.25002 mol and 100 g is 2.5002 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of sodium hydroxide?
2.5002 mol, which is 2500.2 mmol. Read the other way, one mole of sodium hydroxide is 39.99677 g, so 100 g is 2.5 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of sodium hydroxide is 25.002 µmol, 10 mg is 250.02 µmol and 100 mg is 2.5002 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of sodium hydroxide, which is 0.024502 mol rather than 0.025002 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles reference for 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000025 | 0.025002 | 25.002 |
| 5 mg | 0.00012501 | 0.12501 | 125.01 |
| 10 mg | 0.00025002 | 0.25002 | 250.02 |
| 25 mg | 0.00062505 | 0.62505 | 625.05 |
| 50 mg | 0.0012501 | 1.2501 | 1250.1 |
| 100 mg | 0.0025002 | 2.5002 | 2500.2 |
| 250 mg | 0.0062505 | 6.2505 | 6250.5 |
| 500 mg | 0.012501 | 12.501 | 12,501 |
| 1 g | 0.025002 | 25.002 | 25,002 |
| 2 g | 0.050004 | 50.004 | 50,004 |
| 5 g | 0.12501 | 125.01 | 125,010 |
| 10 g | 0.25002 | 250.02 | 250,020 |
| 25 g | 0.62505 | 625.05 | 625,050 |
| 50 g | 1.2501 | 1250.1 | 1250100.968155 |
| 100 g | 2.5002 | 2500.2 | 2500201.936310 |
| 250 g | 6.2505 | 6250.5 | 6250504.840775 |
| 500 g | 12.501 | 12,501 | 12501009.681550 |
Every row is the mass divided by 39.996769 g/mol. A balance reading to 1 mg resolves 25 µmol of this compound, which is the smallest step the middle columns can really move in.
| 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.