Molar MassMolar mass of potassium nitrate (KNO3)
The molar mass of potassium nitrate, KNO3, is 101.10 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 101.10 grams.
101.10 g/mol
39.098 + 14.007 + 3 × 15.999 = 101.102 · 5 atoms in the formula · also called saltpetre, nitre
About potassium nitrate
Potassium nitrate is saltpetre, one of the three ingredients of gunpowder. It is also a fertiliser that supplies both potassium and nitrogen, and it gives off oxygen when heated strongly.
Other names: saltpetre, nitre.
The working
The formula KNO3 has 1 potassium atom, 1 nitrogen atom and 3 oxygen atoms, 5 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Potassium (K) | 1 | 39.098 | 39.098 | 38.67% |
| Nitrogen (N) | 1 | 14.007 | 14.007 | 13.85% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 47.47% |
39.098 + 14.007 + 3 × 15.999 = 101.102 g/mol
With the rounded atomic masses used in many school books (K = 39, N = 14, O = 16) the answer is 101 g/mol. The difference from 101.10 is too small to matter in most school sums.
Percentage composition
By mass, KNO3 is 38.67% potassium, 13.85% nitrogen and 47.47% oxygen. The largest share comes from oxygen. In a 100 g sample there are 38.7 g of potassium, 13.9 g of nitrogen and 47.5 g of oxygen.
Worked examples
Grams to moles. How many moles are in 100 g of KNO3? Divide the mass by the molar mass: 100 ÷ 101.10 = 0.9891 mol.
Moles to grams. What is the mass of 2 mol of KNO3? Multiply: 2 × 101.10 = 202.2 g.
Counting particles. 100 g contains 0.9891 mol × 6.022 × 1023 = 5.957 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 101.10 = 10.11 g of KNO3 in each litre. For 250 mL, weigh 2.528 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 10.11 g |
| 0.5 mol | 50.55 g |
| 1 mol | 101.1 g |
| 2 mol | 202.2 g |
| 5 mol | 505.5 g |
| Mass | Moles |
|---|---|
| 1 g | 0.009891 mol |
| 10 g | 0.09891 mol |
| 100 g | 0.9891 mol |
| 1 kg | 9.891 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of KNO3?
The molar mass of potassium nitrate (KNO3) is 101.10 g/mol, or 101.102 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of KNO3?
100 ÷ 101.10 = 0.9891 mol.
What is the mass of one formula unit of KNO3?
Divide the molar mass by the Avogadro constant: 101.10 ÷ 6.022 × 1023 = 1.679 × 10-22 g.
What is the percentage of oxygen in KNO3?
Oxygen makes up 47.47% of the mass of KNO3: 3 × 15.999 ÷ 101.102 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.