Going from grams to moles for potassium nitrate means dividing by its molar mass, 101.1023 g/mol. One gram is 0.00989097 mol, or 9.89097 mmol; 100 mg is 0.9891 mmol; and a 101.1023 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, KNO3, which is where these figures come from.
The figure comes from the formula. Potassium nitrate is 39.0983 (K) + 14.007 (N) + 3 × 15.999 (O), which is 101.1023 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: 3 of the 5 atoms in a formula unit are oxygen, and they account for 47.997 g of the 101.1023 g, or 47.47% 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 0.09891 mmol of potassium nitrate; on a three-decimal balance it is 1 mg, or 9.891 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.
Saltpetre — a fertiliser, a curing salt and the oxidiser in black powder. Its solubility climbs steeply with temperature, which is how it is purified. At 101.1023 g/mol it is heavier than 10 of the 34 salts covered here, and that ranking matters more than it looks: calcium carbonate has a molar mass of 100.086 g/mol, so a gram of it contains 1.02% more formula units than a gram of potassium nitrate. 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 2.022 g of a nominal 101.1023 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 you weighed, in grams
101.1023- The molar mass of potassium nitrate 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 101.1023 g/mol, the molar mass of potassium nitrate.
- 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 nitrate in moles
0.25 ÷ 101.1023 = 0.00247274 mol, or 2.4727 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 9.891 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of potassium nitrate in moles
2 ÷ 101.1023 = 0.0197819 mol, or 19.782 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 9.891 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of potassium nitrate in moles
25 ÷ 101.1023 = 0.247274 mol, or 247.27 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 9.891 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 nitrate (KNO3)?
It is 101.1023 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 39.0983 (K) + 14.007 (N) + 3 × 15.999 (O). One mole of potassium nitrate therefore weighs 101.1023 g, and a gram of it is 9.891 mmol.
What percentage of potassium nitrate is oxygen?
47.47% by mass. Each formula unit contains 3 oxygen atoms contributing 47.997 g of the 101.1023 g total, so 100 g of potassium nitrate contains 47.47 g of oxygen and a kilogram contains 474.74 g of it.
How do I convert grams of potassium nitrate to moles?
Divide the mass in grams by 101.1023 g/mol. So 1 g is 0.00989097 mol, 10 g is 0.0989097 mol and 100 g is 0.989097 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of potassium nitrate?
0.989097 mol, which is 989.097 mmol. Read the other way, one mole of potassium nitrate is 101.1023 g, so 100 g is 0.9891 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of potassium nitrate is 9.891 µmol, 10 mg is 98.91 µmol and 100 mg is 0.9891 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 nitrate, which is 0.00969315 mol rather than 0.00989097 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 nitrate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 3 | 15.999 | 47.997 | 47.47% |
| Potassium (K) | 1 | 39.0983 | 39.0983 | 38.67% |
| Nitrogen (N) | 1 | 14.007 | 14.007 | 13.85% |
| Total — one mole of potassium nitrate | 101.1023 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 101.1023 g/mol.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000010 | 0.00989097 | 9.89097 |
| 5 mg | 0.000049 | 0.0494549 | 49.4549 |
| 10 mg | 0.000099 | 0.0989097 | 98.9097 |
| 25 mg | 0.000247274 | 0.247274 | 247.274 |
| 50 mg | 0.000494549 | 0.494549 | 494.549 |
| 100 mg | 0.000989097 | 0.989097 | 989.097 |
| 250 mg | 0.00247274 | 2.47274 | 2472.74 |
| 500 mg | 0.00494549 | 4.94549 | 4945.49 |
| 1 g | 0.00989097 | 9.89097 | 9890.97 |
| 2 g | 0.0197819 | 19.7819 | 19,781.9 |
| 5 g | 0.0494549 | 49.4549 | 49,454.9 |
| 10 g | 0.0989097 | 98.9097 | 98,909.7 |
| 25 g | 0.247274 | 247.274 | 247,274 |
| 50 g | 0.494549 | 494.549 | 494,549 |
| 100 g | 0.989097 | 989.097 | 989,097 |
| 250 g | 2.47274 | 2472.74 | 2472742.954414 |
| 500 g | 4.94549 | 4945.49 | 4945485.908827 |
Every row is the mass divided by 101.1023 g/mol. A balance reading to 1 mg resolves 9.891 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 nitrate | NaNO3 | 84.99377 | 11.766 |
| Magnesium chloride | MgCl2 | 95.205 | 10.504 |
| Calcium carbonate | CaCO3 | 100.086 | 9.9914 |
| Potassium nitrate (this page) | KNO3 | 101.1023 | 9.891 |
| Sodium carbonate | Na2CO3 | 105.9875 | 9.4351 |
| Calcium chloride | CaCl2 | 110.978 | 9.0108 |
| Magnesium sulfate | MgSO4 | 120.361 | 8.3083 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.