Going from grams to moles for potassium permanganate means dividing by its molar mass, 158.03234 g/mol. One gram is 0.00632782 mol, or 6.32782 mmol; 100 mg is 0.63278 mmol; and a 158.0323 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, KMnO4, which is where these figures come from.
The figure comes from the formula. Potassium permanganate is 39.0983 (K) + 54.938 (Mn) + 4 × 15.999 (O), which is 158.03234 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 4 of the 6 atoms in a formula unit are oxygen, and they account for 63.996 g of the 158.0323 g, or 40.5% 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 0.06328 mmol of potassium permanganate; on a three-decimal balance it is 1 mg, or 6.328 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.
A strong oxidiser used in water treatment and titration. Its solutions are intensely purple and stain everything, and they slowly decompose in light. At 158.0323 g/mol it is heavier than 18 of the 34 salts covered here, and that ranking matters more than it looks: sodium sulfate has a molar mass of 142.0355 g/mol, so a gram of it contains 11.3% more formula units than a gram of potassium permanganate. 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, 3.1606 g of every 158.0323 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
158.03234- The molar mass of potassium permanganate 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 158.03234 g/mol, the molar mass of potassium permanganate.
- 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 permanganate in moles
0.25 ÷ 158.0323 = 0.00158195 mol, or 1.582 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.328 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of potassium permanganate in moles
2 ÷ 158.0323 = 0.0126556 mol, or 12.656 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.328 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of potassium permanganate in moles
25 ÷ 158.0323 = 0.158195 mol, or 158.2 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.328 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 permanganate (KMnO4)?
It is 158.03234 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 39.0983 (K) + 54.938 (Mn) + 4 × 15.999 (O). One mole of potassium permanganate therefore weighs 158.0323 g, and a gram of it is 6.3278 mmol.
What percentage of potassium permanganate is oxygen?
40.5% by mass. Each formula unit contains 4 oxygen atoms contributing 63.996 g of the 158.0323 g total, so 100 g of potassium permanganate contains 40.5 g of oxygen and a kilogram contains 404.96 g of it.
How do I convert grams of potassium permanganate to moles?
Divide the mass in grams by 158.03234 g/mol. So 1 g is 0.00632782 mol, 10 g is 0.0632782 mol and 100 g is 0.632782 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of potassium permanganate?
0.632782 mol, which is 632.782 mmol. Read the other way, one mole of potassium permanganate is 158.0323 g, so 100 g is 0.6328 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of potassium permanganate is 6.3278 µmol, 10 mg is 63.278 µmol and 100 mg is 0.63278 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 permanganate, which is 0.00620126 mol rather than 0.00632782 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 permanganate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 4 | 15.999 | 63.996 | 40.5% |
| Manganese (Mn) | 1 | 54.93804 | 54.93804 | 34.76% |
| Potassium (K) | 1 | 39.0983 | 39.0983 | 24.74% |
| Total — one mole of potassium permanganate | 158.03234 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 158.03234 g/mol.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000006 | 0.00632782 | 6.32782 |
| 5 mg | 0.000032 | 0.0316391 | 31.6391 |
| 10 mg | 0.000063 | 0.0632782 | 63.2782 |
| 25 mg | 0.000158195 | 0.158195 | 158.195 |
| 50 mg | 0.000316391 | 0.316391 | 316.391 |
| 100 mg | 0.000632782 | 0.632782 | 632.782 |
| 250 mg | 0.00158195 | 1.58195 | 1581.95 |
| 500 mg | 0.00316391 | 3.16391 | 3163.91 |
| 1 g | 0.00632782 | 6.32782 | 6327.82 |
| 2 g | 0.0126556 | 12.6556 | 12,655.6 |
| 5 g | 0.0316391 | 31.6391 | 31,639.1 |
| 10 g | 0.0632782 | 63.2782 | 63,278.2 |
| 25 g | 0.158195 | 158.195 | 158,195 |
| 50 g | 0.316391 | 316.391 | 316,391 |
| 100 g | 0.632782 | 632.782 | 632,782 |
| 250 g | 1.58195 | 1581.95 | 1581954.650891 |
| 500 g | 3.16391 | 3163.91 | 3163909.301781 |
Every row is the mass divided by 158.03234 g/mol. A balance reading to 1 mg resolves 6.328 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 |
|---|---|---|---|
| Monopotassium phosphate | KH2PO4 | 136.0841 | 7.3484 |
| Disodium phosphate | Na2HPO4 | 141.9573 | 7.0444 |
| Sodium sulfate | Na2SO4 | 142.0355 | 7.0405 |
| Potassium permanganate (this page) | KMnO4 | 158.0323 | 6.3278 |
| Copper(II) sulfate | CuSO4 | 159.602 | 6.2656 |
| Iron(III) chloride | FeCl3 | 162.195 | 6.1654 |
| Trisodium phosphate | Na3PO4 | 163.9391 | 6.0998 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.