Percentage strength for potassium nitrate is read as % w/v: grams of solute in 100 mL of solution. So 1% is 1 g per 100 mL, which for this compound works out at 0.09891 mol/L, and 0.9% is 9 g per litre or 0.089019 mol/L. The calculator takes a percentage and a volume and returns the grams to weigh, with the molarity and mg/mL equivalents beside it.
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.
Three different percentages are in circulation and they are not interchangeable. Per cent w/v is mass in a volume, the one used here and on medical labels; per cent w/w is mass in mass, the one on drums of concentrated reagent; per cent v/v is volume in volume, used for liquids mixed into liquids. For a dilute aqueous solution w/v and w/w are within a percent of each other, but for anything concentrated the density has to come into it.
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
p- The strength you want, as % w/v
V- The volume of solution, in millilitres
m- The mass of potassium nitrate to weigh out, in grams
101.1023- The molar mass of potassium nitrate in g/mol, derived from its formula
How it works, step by step
- Enter the percentage strength you want, as % w/v.
- Enter the volume of solution in millilitres.
- Grams needed is percentage × volume ÷ 100, since 1% w/v is 1 g per 100 mL.
- The molarity that strength corresponds to for potassium nitrate is shown beneath, using 101.1023 g/mol.
Worked examples
0.9% w/v potassium nitrate in 500 mL
0.9% w/v is 0.9 g per 100 mL, so 500 mL needs 4.5 g. In molar terms that solution is 0.089019 mol/L, and it is 9 mg/mL if you are dosing by volume.
5% w/v potassium nitrate in 100 mL
5% w/v is 5 g per 100 mL, so 100 mL needs 5 g. In molar terms that solution is 0.49455 mol/L, and it is 50 mg/mL if you are dosing by volume.
10% w/v potassium nitrate in 1000 mL
10% w/v is 10 g per 100 mL, so 1000 mL needs 100 g. In molar terms that solution is 0.9891 mol/L, and it is 100 mg/mL if you are dosing by volume.
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 of potassium nitrate make a 1% w/v solution?
1 g in 100 mL of solution, 5 g in 500 mL, 10 g in a litre. The percentage is fixed to the volume, not to the compound, so those masses are the same whatever is being dissolved — what changes with the compound is the molarity they come to.
What molarity is a 1% w/v solution of potassium nitrate?
0.0989097 mol/L, because 1% w/v is 10 g/L and 101.1023 g/L is one molar. A 0.9% solution is 0.089019 mol/L and a 5% solution is 0.49455 mol/L.
Is % w/v the same as % w/w?
No. Per cent w/v is grams per 100 mL of solution; per cent w/w is grams per 100 g of solution. They coincide only when the solution has a density of 1.00 g/mL, which dilute aqueous solutions approximately do and concentrated ones do not. Labels on concentrated reagents are usually w/w.
What is 1% w/v in mg/mL?
10 mg/mL, since 1 g in 100 mL is 1000 mg in 100 mL. For potassium nitrate that is also 98.91 mmol/L, which is the form a dose calculation usually wants.
Percentage strengths of 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.
| Strength | per 100 mL | per 500 mL | per 1 L | Molarity (mol/L) |
|---|---|---|---|---|
| 0.1% w/v | 100 mg | 500 mg | 1 g | 0.009891 |
| 0.25% w/v | 250 mg | 1.25 g | 2.5 g | 0.024727 |
| 0.5% w/v | 500 mg | 2.5 g | 5 g | 0.049455 |
| 0.9% w/v | 900 mg | 4.5 g | 9 g | 0.089019 |
| 1% w/v | 1 g | 5 g | 10 g | 0.09891 |
| 2% w/v | 2 g | 10 g | 20 g | 0.19782 |
| 3% w/v | 3 g | 15 g | 30 g | 0.29673 |
| 5% w/v | 5 g | 25 g | 50 g | 0.49455 |
| 10% w/v | 10 g | 50 g | 100 g | 0.9891 |
| 20% w/v | 20 g | 100 g | 200 g | 1.9782 |
A 1% w/v solution is 1 g in 100 mL, which for this compound is 0.09891 mol/L because 10 g/L divided by 101.1023 g/mol gives that figure.
| 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.