To prepare a solution of sodium hydroxide you need the grams, and grams is molarity × volume in litres × 39.996769 g/mol. A litre of 1 M takes 39.99677 g; 500 mL of 0.1 M takes 1.9998 g; 250 mL of 0.01 M takes 99.992 mg. Enter the strength you want and the volume you want it in, and the calculator returns the mass to weigh together with the same figure in milligrams and as a percentage strength.
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.
Order of operations matters more than precision here. Dissolve the solid in rather less than the final volume, then make up to the mark once it has all gone into solution and come back to room temperature — warm solutions read low when they cool. If the calculated mass is under about 20 mg, prepare ten times the strength and dilute it tenfold by pipette instead, because C₁V₁ = C₂V₂ carries less error than a marginal 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.
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
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
39.996769- The molar mass of sodium hydroxide in g/mol, derived from its formula
m- The mass of sodium hydroxide to weigh out, in grams
How it works, step by step
- Enter the molarity you want, in mol/L.
- Enter the volume you want to make, in millilitres.
- The two are multiplied together and by 39.996769 g/mol.
- The result is the mass to weigh; dissolve it in part of the volume and then make up to the mark.
Worked examples
250 mL of 0.1 M sodium hydroxide
Weigh 0.999919 g. That is 0.1 M × 0.25 L × 39.99677 g/mol, and it comes to 999.919 mg if your balance is set to milligrams. Dissolve in about 175 mL first, then make up to 250 mL.
500 mL of 0.5 M sodium hydroxide
Weigh 9.99919 g. That is 0.5 M × 0.5 L × 39.99677 g/mol, and it comes to 9999.19 mg if your balance is set to milligrams. Dissolve in about 350 mL first, then make up to 500 mL.
1000 mL of 1 M sodium hydroxide
Weigh 39.9968 g. That is 1 M × 1 L × 39.99677 g/mol, and it comes to 39,996.8 mg if your balance is set to milligrams. Dissolve in about 700 mL first, then make up to 1000 mL.
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 much sodium hydroxide do I need for 1 litre of 1 M solution?
39.996769 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 3.99968 g, and for 1 L of 0.01 M it is 0.399968 g.
How much for 100 mL of 0.1 M?
0.399968 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 39.99677 g. In milligrams that is 399.968 mg.
Do I dissolve the solid first or make up the volume first?
Dissolve first, in perhaps 70% of the final volume, then make up to the mark. Adding solid to a full flask overshoots the volume and leaves the solution weak, and undissolved solid at the mark means the strength keeps changing as it goes in.
Can I dilute a stock solution instead of weighing?
Yes, and it is more accurate for small amounts. C₁V₁ = C₂V₂: to get 500 mL of 0.01 M from a 0.5 M stock, take 10 mL of stock and make up to 500 mL. Weighing the 199.98 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes 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.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 3.99968 mg | 9.99919 mg | 19.9984 mg | 39.9968 mg |
| 0.005 M | 19.9984 mg | 49.996 mg | 99.9919 mg | 199.984 mg |
| 0.01 M | 39.9968 mg | 99.9919 mg | 199.984 mg | 399.968 mg |
| 0.05 M | 199.984 mg | 499.96 mg | 999.919 mg | 1.99984 g |
| 0.1 M | 399.968 mg | 999.919 mg | 1.99984 g | 3.99968 g |
| 0.15 M | 599.952 mg | 1.49988 g | 2.99976 g | 5.99952 g |
| 0.2 M | 799.935 mg | 1.99984 g | 3.99968 g | 7.99935 g |
| 0.25 M | 999.919 mg | 2.4998 g | 4.9996 g | 9.99919 g |
| 0.5 M | 1.99984 g | 4.9996 g | 9.99919 g | 19.9984 g |
| 1 M | 3.99968 g | 9.99919 g | 19.9984 g | 39.9968 g |
| 2 M | 7.99935 g | 19.9984 g | 39.9968 g | 79.9935 g |
Each cell is molarity × volume in litres × 39.996769 g/mol. Dissolve the solid first and then make up to the marked volume — adding solid to a full volume overshoots it.
| 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.