Molar MassMolar mass of potassium hydroxide (KOH)
The molar mass of potassium hydroxide, KOH, is 56.10 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 56.10 grams.
56.10 g/mol
39.098 + 15.999 + 1.0080 = 56.105 · 3 atoms in the formula · also called caustic potash
About potassium hydroxide
Potassium hydroxide is a strong alkali much like sodium hydroxide, but heavier per mole because potassium is heavier than sodium. It is used for soft and liquid soaps and as the electrolyte in alkaline batteries.
Other names: caustic potash.
The working
The formula KOH has 1 potassium atom, 1 oxygen atom and 1 hydrogen atom, 3 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Potassium (K) | 1 | 39.098 | 39.098 | 69.69% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 28.52% |
| Hydrogen (H) | 1 | 1.0080 | 1.008 | 1.80% |
39.098 + 15.999 + 1.0080 = 56.105 g/mol
With the rounded atomic masses used in many school books (K = 39, O = 16, H = 1) the answer is 56 g/mol. The difference from 56.10 is too small to matter in most school sums.
Percentage composition
By mass, KOH is 69.69% potassium, 28.52% oxygen and 1.80% hydrogen. The largest share comes from potassium. In a 100 g sample there are 69.7 g of potassium, 28.5 g of oxygen and 1.8 g of hydrogen.
Worked examples
Grams to moles. How many moles are in 250 g of KOH? Divide the mass by the molar mass: 250 ÷ 56.10 = 4.456 mol.
Moles to grams. What is the mass of 0.2 mol of KOH? Multiply: 0.2 × 56.10 = 11.22 g.
Counting particles. 250 g contains 4.456 mol × 6.022 × 1023 = 2.683 × 1024 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 56.10 = 5.611 g of KOH in each litre. For 250 mL, weigh 1.403 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 5.611 g |
| 0.5 mol | 28.05 g |
| 1 mol | 56.1 g |
| 2 mol | 112.2 g |
| 5 mol | 280.5 g |
| Mass | Moles |
|---|---|
| 1 g | 0.01782 mol |
| 10 g | 0.1782 mol |
| 100 g | 1.782 mol |
| 1 kg | 17.82 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of KOH?
The molar mass of potassium hydroxide (KOH) is 56.10 g/mol, or 56.105 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of KOH?
100 ÷ 56.10 = 1.782 mol.
What is the mass of one formula unit of KOH?
Divide the molar mass by the Avogadro constant: 56.10 ÷ 6.022 × 1023 = 9.316 × 10-23 g.
What is the percentage of potassium in KOH?
Potassium makes up 69.69% of the mass of KOH: 1 × 39.098 ÷ 56.105 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.