To prepare a solution of potassium hydroxide you need the grams, and grams is molarity × volume in litres × 56.1053 g/mol. A litre of 1 M takes 56.1053 g; 500 mL of 0.1 M takes 2.8053 g; 250 mL of 0.01 M takes 140.26 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. Potassium hydroxide is 39.0983 (K) + 15.999 (O) + 1.008 (H), which is 56.1053 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Potassium makes up the largest share of the mass: 1 of the 3 atoms in a formula unit is potassium, and they account for 39.0983 g of the 56.1053 g, or 69.69% 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 potash, the base used for liquid soap and in alkaline batteries. Like sodium hydroxide it is hygroscopic and cannot be weighed at leisure. At 56.1053 g/mol it is heavier than 1 of the 3 bases covered here, and that ranking matters more than it looks: sodium hydroxide has a molar mass of 39.99677 g/mol, so a gram of it contains 40.3% more formula units than a gram of potassium 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 1.1221 g of a nominal 56.1053 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
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
56.1053- The molar mass of potassium hydroxide in g/mol, derived from its formula
m- The mass of potassium 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 56.1053 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 potassium hydroxide
Weigh 1.40263 g. That is 0.1 M × 0.25 L × 56.1053 g/mol, and it comes to 1402.63 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 potassium hydroxide
Weigh 14.0263 g. That is 0.5 M × 0.5 L × 56.1053 g/mol, and it comes to 14,026.3 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 potassium hydroxide
Weigh 56.1053 g. That is 1 M × 1 L × 56.1053 g/mol, and it comes to 56,105.3 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 potassium hydroxide (KOH)?
It is 56.1053 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 39.0983 (K) + 15.999 (O) + 1.008 (H). One mole of potassium hydroxide therefore weighs 56.1053 g, and a gram of it is 17.824 mmol.
What percentage of potassium hydroxide is potassium?
69.69% by mass. Each formula unit contains 1 potassium atom contributing 39.0983 g of the 56.1053 g total, so 100 g of potassium hydroxide contains 69.69 g of potassium and a kilogram contains 696.87 g of it.
How much potassium hydroxide do I need for 1 litre of 1 M solution?
56.1053 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 5.61053 g, and for 1 L of 0.01 M it is 0.561053 g.
How much for 100 mL of 0.1 M?
0.561053 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 56.1053 g. In milligrams that is 561.053 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 280.53 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes for potassium hydroxide
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Potassium (K) | 1 | 39.0983 | 39.0983 | 69.69% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 28.52% |
| Hydrogen (H) | 1 | 1.008 | 1.008 | 1.797% |
| Total — one mole of potassium hydroxide | 56.1053 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 56.1053 g/mol.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 5.61053 mg | 14.0263 mg | 28.0527 mg | 56.1053 mg |
| 0.005 M | 28.0527 mg | 70.1316 mg | 140.263 mg | 280.527 mg |
| 0.01 M | 56.1053 mg | 140.263 mg | 280.527 mg | 561.053 mg |
| 0.05 M | 280.527 mg | 701.316 mg | 1.40263 g | 2.80527 g |
| 0.1 M | 561.053 mg | 1.40263 g | 2.80527 g | 5.61053 g |
| 0.15 M | 841.58 mg | 2.10395 g | 4.2079 g | 8.4158 g |
| 0.2 M | 1.12211 g | 2.80527 g | 5.61053 g | 11.2211 g |
| 0.25 M | 1.40263 g | 3.50658 g | 7.01316 g | 14.0263 g |
| 0.5 M | 2.80527 g | 7.01316 g | 14.0263 g | 28.0527 g |
| 1 M | 5.61053 g | 14.0263 g | 28.0527 g | 56.1053 g |
| 2 M | 11.2211 g | 28.0527 g | 56.1053 g | 112.211 g |
Each cell is molarity × volume in litres × 56.1053 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 | NaOH | 39.99677 | 25.002 |
| Potassium hydroxide (this page) | 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.