The molar mass of potassium hydroxide, KOH, is 56.1053 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 56.1053 g, and a gram of it is 1.073e+22 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside 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.
The numbers a bench actually uses sit at the millimole scale. One millimole of potassium hydroxide is 56.1053 mg and one micromole is 56.1053 µg, so a balance reading to a milligram places you within 17.82 µmol. A single formula unit weighs 9.3165e-23 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
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
m- The mass of potassium hydroxide in grams
56.1053- The molar mass of potassium hydroxide in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of potassium hydroxide in grams.
- It is divided by the molar mass, 56.1053 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of potassium hydroxide as a molecule count
0.5 g divided by 56.1053 g/mol is 0.00891181 mol, and multiplying by the Avogadro constant gives 5.367e+21 formula units of potassium hydroxide. The same mass is 8.9118 mmol, which is the figure a reaction is actually planned in.
5 g of potassium hydroxide as a molecule count
5 g divided by 56.1053 g/mol is 0.0891181 mol, and multiplying by the Avogadro constant gives 5.367e+22 formula units of potassium hydroxide. The same mass is 89.118 mmol, which is the figure a reaction is actually planned in.
50 g of potassium hydroxide as a molecule count
50 g divided by 56.1053 g/mol is 0.891181 mol, and multiplying by the Avogadro constant gives 5.367e+23 formula units of potassium hydroxide. The same mass is 891.18 mmol, which is the figure a reaction is actually planned in.
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 molecules are in one gram of potassium hydroxide?
About 1.073e+22. One gram is 0.0178236 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 56.1053.
How much does a single molecule of potassium hydroxide weigh?
9.3165e-23 g, which is 56.1053 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for potassium hydroxide it is 56.1053. Molar mass carries grams per mole: 56.1053 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Potassium is quoted as 39.0983 rather than a whole number for that reason, and the same applies to the other elements here, which is why potassium hydroxide comes out at 56.1053 g/mol rather than 56.
Molar mass, composition and mole equivalents 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.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000056 | 0.0561053 | 6.022e+17 |
| 10 µmol | 0.000561053 | 0.561053 | 6.022e+18 |
| 100 µmol | 0.00561053 | 5.61053 | 6.022e+19 |
| 1 mmol | 0.0561053 | 56.1053 | 6.022e+20 |
| 10 mmol | 0.561053 | 561.053 | 6.022e+21 |
| 50 mmol | 2.80527 | 2805.27 | 3.011e+22 |
| 100 mmol | 5.61053 | 5610.53 | 6.022e+22 |
| 250 mmol | 14.0263 | 14,026.3 | 1.506e+23 |
| 500 mmol | 28.0527 | 28,052.7 | 3.011e+23 |
| 750 mmol | 42.079 | 42,079 | 4.517e+23 |
| 1 mol | 56.1053 | 56,105.3 | 6.022e+23 |
| 2 mol | 112.211 | 112,211 | 1.204e+24 |
| 5 mol | 280.527 | 280,527 | 3.011e+24 |
| 10 mol | 561.053 | 561,053 | 6.022e+24 |
Mass is the amount multiplied by 56.1053 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| 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.