Moles to grams for potassium hydroxide is a multiplication by 56.1053 g/mol. A mole weighs 56.1053 g, 100 mmol weighs 5.61053 g, 10 mmol weighs 0.561053 g and a millimole is 56.1053 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
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.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
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
n- The amount of potassium hydroxide you need, in moles
56.1053- The molar mass of potassium hydroxide in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 56.1053 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of potassium hydroxide in grams
5 mmol × 56.1053 g/mol = 0.280527 g, which is 280.527 mg. Doubling the amount doubles the mass to 0.561053 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of potassium hydroxide in grams
250 mmol × 56.1053 g/mol = 14.0263 g, which is 14,026.3 mg. Doubling the amount doubles the mass to 28.0527 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of potassium hydroxide in grams
2 mol × 56.1053 g/mol = 112.211 g, which is 112,211 mg. Doubling the amount doubles the mass to 224.421 g, because the relationship is a plain proportion once the molar mass is fixed.
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 is one mole of potassium hydroxide?
56.1053 g, by definition of the molar mass. Two moles is 112.2106 g and half a mole is 28.05265 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of potassium hydroxide is 5.61053 g. One millimole is 56.1053 mg and one micromole is 56.1053 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 0.5611 mg of potassium hydroxide, weigh 5.611 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the anhydrous form, KOH, at 56.1053 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. Salts that are sold as hydrates weigh more per mole because the water is part of the crystal.
Moles to grams reference 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000056 | 0.0561053 | 0.000057 |
| 10 µmol | 0.000561053 | 0.561053 | 0.000572274 |
| 100 µmol | 0.00561053 | 5.61053 | 0.00572274 |
| 1 mmol | 0.0561053 | 56.1053 | 0.0572274 |
| 10 mmol | 0.561053 | 561.053 | 0.572274 |
| 50 mmol | 2.80527 | 2805.27 | 2.86137 |
| 100 mmol | 5.61053 | 5610.53 | 5.72274 |
| 250 mmol | 14.0263 | 14,026.3 | 14.3069 |
| 500 mmol | 28.0527 | 28,052.7 | 28.6137 |
| 750 mmol | 42.079 | 42,079 | 42.9206 |
| 1 mol | 56.1053 | 56,105.3 | 57.2274 |
| 2 mol | 112.211 | 112,211 | 114.455 |
| 5 mol | 280.527 | 280,527 | 286.137 |
| 10 mol | 561.053 | 561,053 | 572.274 |
Weigh is the amount multiplied by 56.1053 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| 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.