Going from grams to moles for potassium hydroxide means dividing by its molar mass, 56.1053 g/mol. One gram is 0.0178236 mol, or 17.8236 mmol; 100 mg is 1.7824 mmol; and a 56.1053 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, KOH, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.1782 mmol of potassium hydroxide; on a three-decimal balance it is 1 mg, or 17.82 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats 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.
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
m- The mass you weighed, in grams
56.1053- The molar mass of potassium hydroxide in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 56.1053 g/mol, the molar mass of potassium hydroxide.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of potassium hydroxide in moles
0.25 ÷ 56.1053 = 0.00445591 mol, or 4.4559 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 17.82 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of potassium hydroxide in moles
2 ÷ 56.1053 = 0.0356473 mol, or 35.647 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 17.82 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of potassium hydroxide in moles
25 ÷ 56.1053 = 0.445591 mol, or 445.59 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 17.82 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
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 do I convert grams of potassium hydroxide to moles?
Divide the mass in grams by 56.1053 g/mol. So 1 g is 0.0178236 mol, 10 g is 0.178236 mol and 100 g is 1.78236 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of potassium hydroxide?
1.78236 mol, which is 1782.36 mmol. Read the other way, one mole of potassium hydroxide is 56.1053 g, so 100 g is 1.782 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of potassium hydroxide is 17.824 µmol, 10 mg is 178.24 µmol and 100 mg is 1.7824 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of potassium hydroxide, which is 0.0174672 mol rather than 0.0178236 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000018 | 0.0178236 | 17.8236 |
| 5 mg | 0.000089 | 0.0891181 | 89.1181 |
| 10 mg | 0.000178236 | 0.178236 | 178.236 |
| 25 mg | 0.000445591 | 0.445591 | 445.591 |
| 50 mg | 0.000891181 | 0.891181 | 891.181 |
| 100 mg | 0.00178236 | 1.78236 | 1782.36 |
| 250 mg | 0.00445591 | 4.45591 | 4455.91 |
| 500 mg | 0.00891181 | 8.91181 | 8911.81 |
| 1 g | 0.0178236 | 17.8236 | 17,823.6 |
| 2 g | 0.0356473 | 35.6473 | 35,647.3 |
| 5 g | 0.0891181 | 89.1181 | 89,118.1 |
| 10 g | 0.178236 | 178.236 | 178,236 |
| 25 g | 0.445591 | 445.591 | 445,591 |
| 50 g | 0.891181 | 891.181 | 891,181 |
| 100 g | 1.78236 | 1782.36 | 1782362.807079 |
| 250 g | 4.45591 | 4455.91 | 4455907.017697 |
| 500 g | 8.91181 | 8911.81 | 8911814.035394 |
Every row is the mass divided by 56.1053 g/mol. A balance reading to 1 mg resolves 17.82 µmol of this compound, which is the smallest step the middle columns can really move in.
| 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.