Moles to grams for sodium hydroxide is a multiplication by 39.996769 g/mol. A mole weighs 39.99677 g, 100 mmol weighs 3.99968 g, 10 mmol weighs 0.399968 g and a millimole is 39.9968 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
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.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
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
n- The amount of sodium hydroxide you need, in moles
39.996769- The molar mass of sodium hydroxide in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 39.996769 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of sodium hydroxide in grams
5 mmol × 39.99677 g/mol = 0.199984 g, which is 199.984 mg. Doubling the amount doubles the mass to 0.399968 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of sodium hydroxide in grams
250 mmol × 39.99677 g/mol = 9.99919 g, which is 9999.19 mg. Doubling the amount doubles the mass to 19.9984 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of sodium hydroxide in grams
2 mol × 39.99677 g/mol = 79.9935 g, which is 79,993.5 mg. Doubling the amount doubles the mass to 159.987 g, because the relationship is a plain proportion once the molar mass is fixed.
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 many grams is one mole of sodium hydroxide?
39.996769 g, by definition of the molar mass. Two moles is 79.99354 g and half a mole is 19.99838 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of sodium hydroxide is 3.99968 g. One millimole is 39.9968 mg and one micromole is 39.9968 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 0.4 mg of sodium hydroxide, weigh 4 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the anhydrous form, NaOH, at 39.996769 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. Salts that are sold as hydrates weigh more per mole because the water is part of the crystal.
Moles to grams 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000040 | 0.0399968 | 0.000041 |
| 10 µmol | 0.000399968 | 0.399968 | 0.000407967 |
| 100 µmol | 0.00399968 | 3.99968 | 0.00407967 |
| 1 mmol | 0.0399968 | 39.9968 | 0.0407967 |
| 10 mmol | 0.399968 | 399.968 | 0.407967 |
| 50 mmol | 1.99984 | 1999.84 | 2.03984 |
| 100 mmol | 3.99968 | 3999.68 | 4.07967 |
| 250 mmol | 9.99919 | 9999.19 | 10.1992 |
| 500 mmol | 19.9984 | 19,998.4 | 20.3984 |
| 750 mmol | 29.9976 | 29,997.6 | 30.5975 |
| 1 mol | 39.9968 | 39,996.8 | 40.7967 |
| 2 mol | 79.9935 | 79,993.5 | 81.5934 |
| 5 mol | 199.984 | 199,984 | 203.984 |
| 10 mol | 399.968 | 399,968 | 407.967 |
Weigh is the amount multiplied by 39.996769 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| 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.