Moles to grams for potassium nitrate is a multiplication by 101.1023 g/mol. A mole weighs 101.1023 g, 100 mmol weighs 10.1102 g, 10 mmol weighs 1.01102 g and a millimole is 101.102 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 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.
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.
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.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 2.022 g of every 101.1023 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
n- The amount of potassium nitrate you need, in moles
101.1023- The molar mass of potassium nitrate 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 101.1023 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 nitrate in grams
5 mmol × 101.1023 g/mol = 0.505512 g, which is 505.512 mg. Doubling the amount doubles the mass to 1.01102 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of potassium nitrate in grams
250 mmol × 101.1023 g/mol = 25.2756 g, which is 25,275.6 mg. Doubling the amount doubles the mass to 50.5511 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of potassium nitrate in grams
2 mol × 101.1023 g/mol = 202.205 g, which is 202,205 mg. Doubling the amount doubles the mass to 404.409 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 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 many grams is one mole of potassium nitrate?
101.1023 g, by definition of the molar mass. Two moles is 202.2046 g and half a mole is 50.55115 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of potassium nitrate is 10.1102 g. One millimole is 101.102 mg and one micromole is 101.102 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 1.011 mg of potassium nitrate, weigh 10.11 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, KNO3, at 101.1023 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 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000101102 | 0.101102 | 0.000103124 |
| 10 µmol | 0.00101102 | 1.01102 | 0.00103124 |
| 100 µmol | 0.0101102 | 10.1102 | 0.0103124 |
| 1 mmol | 0.101102 | 101.102 | 0.103124 |
| 10 mmol | 1.01102 | 1011.02 | 1.03124 |
| 50 mmol | 5.05512 | 5055.12 | 5.15622 |
| 100 mmol | 10.1102 | 10,110.2 | 10.3124 |
| 250 mmol | 25.2756 | 25,275.6 | 25.7811 |
| 500 mmol | 50.5511 | 50,551.2 | 51.5622 |
| 750 mmol | 75.8267 | 75,826.7 | 77.3433 |
| 1 mol | 101.102 | 101,102 | 103.124 |
| 2 mol | 202.205 | 202,205 | 206.249 |
| 5 mol | 505.512 | 505,512 | 515.622 |
| 10 mol | 1011.02 | 1011023.000000 | 1031.24 |
Weigh is the amount multiplied by 101.1023 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 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.