Percentage strength for potassium hydroxide is read as % w/v: grams of solute in 100 mL of solution. So 1% is 1 g per 100 mL, which for this compound works out at 0.17824 mol/L, and 0.9% is 9 g per litre or 0.16041 mol/L. The calculator takes a percentage and a volume and returns the grams to weigh, with the molarity and mg/mL equivalents beside it.
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.
Three different percentages are in circulation and they are not interchangeable. Per cent w/v is mass in a volume, the one used here and on medical labels; per cent w/w is mass in mass, the one on drums of concentrated reagent; per cent v/v is volume in volume, used for liquids mixed into liquids. For a dilute aqueous solution w/v and w/w are within a percent of each other, but for anything concentrated the density has to come into it.
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.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 1.1221 g of every 56.1053 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
p- The strength you want, as % w/v
V- The volume of solution, in millilitres
m- The mass of potassium hydroxide to weigh out, in grams
56.1053- The molar mass of potassium hydroxide in g/mol, derived from its formula
How it works, step by step
- Enter the percentage strength you want, as % w/v.
- Enter the volume of solution in millilitres.
- Grams needed is percentage × volume ÷ 100, since 1% w/v is 1 g per 100 mL.
- The molarity that strength corresponds to for potassium hydroxide is shown beneath, using 56.1053 g/mol.
Worked examples
0.9% w/v potassium hydroxide in 500 mL
0.9% w/v is 0.9 g per 100 mL, so 500 mL needs 4.5 g. In molar terms that solution is 0.16041 mol/L, and it is 9 mg/mL if you are dosing by volume.
5% w/v potassium hydroxide in 100 mL
5% w/v is 5 g per 100 mL, so 100 mL needs 5 g. In molar terms that solution is 0.89118 mol/L, and it is 50 mg/mL if you are dosing by volume.
10% w/v potassium hydroxide in 1000 mL
10% w/v is 10 g per 100 mL, so 1000 mL needs 100 g. In molar terms that solution is 1.7824 mol/L, and it is 100 mg/mL if you are dosing by volume.
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 many grams of potassium hydroxide make a 1% w/v solution?
1 g in 100 mL of solution, 5 g in 500 mL, 10 g in a litre. The percentage is fixed to the volume, not to the compound, so those masses are the same whatever is being dissolved — what changes with the compound is the molarity they come to.
What molarity is a 1% w/v solution of potassium hydroxide?
0.178236 mol/L, because 1% w/v is 10 g/L and 56.1053 g/L is one molar. A 0.9% solution is 0.16041 mol/L and a 5% solution is 0.89118 mol/L.
Is % w/v the same as % w/w?
No. Per cent w/v is grams per 100 mL of solution; per cent w/w is grams per 100 g of solution. They coincide only when the solution has a density of 1.00 g/mL, which dilute aqueous solutions approximately do and concentrated ones do not. Labels on concentrated reagents are usually w/w.
What is 1% w/v in mg/mL?
10 mg/mL, since 1 g in 100 mL is 1000 mg in 100 mL. For potassium hydroxide that is also 178.24 mmol/L, which is the form a dose calculation usually wants.
Percentage strengths of 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.
| Strength | per 100 mL | per 500 mL | per 1 L | Molarity (mol/L) |
|---|---|---|---|---|
| 0.1% w/v | 100 mg | 500 mg | 1 g | 0.017824 |
| 0.25% w/v | 250 mg | 1.25 g | 2.5 g | 0.044559 |
| 0.5% w/v | 500 mg | 2.5 g | 5 g | 0.089118 |
| 0.9% w/v | 900 mg | 4.5 g | 9 g | 0.16041 |
| 1% w/v | 1 g | 5 g | 10 g | 0.17824 |
| 2% w/v | 2 g | 10 g | 20 g | 0.35647 |
| 3% w/v | 3 g | 15 g | 30 g | 0.53471 |
| 5% w/v | 5 g | 25 g | 50 g | 0.89118 |
| 10% w/v | 10 g | 50 g | 100 g | 1.7824 |
| 20% w/v | 20 g | 100 g | 200 g | 3.5647 |
A 1% w/v solution is 1 g in 100 mL, which for this compound is 0.17824 mol/L because 10 g/L divided by 56.1053 g/mol gives that figure.
| 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.