Percentage strength for potassium iodide 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 0.06024 mol/L, and 0.9% is 9 g per litre or 0.054216 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 iodide is 39.0983 (K) + 126.904 (I), which is 166.00277 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Iodine makes up the largest share of the mass: 1 of the 2 atoms in a formula unit is iodine, and they account for 126.904 g of the 166.0028 g, or 76.45% 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.
The iodine in iodised salt, the reagent in starch tests and the thyroid blocking agent. Solutions turn yellow as iodide oxidises to iodine. At 166.0028 g/mol it is heavier than 22 of the 34 salts covered here, and that ranking matters more than it looks: trisodium phosphate has a molar mass of 163.9391 g/mol, so a gram of it contains 1.26% more formula units than a gram of potassium iodide. 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 3.3201 g of a nominal 166.0028 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
p- The strength you want, as % w/v
V- The volume of solution, in millilitres
m- The mass of potassium iodide to weigh out, in grams
166.00277- The molar mass of potassium iodide 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 iodide is shown beneath, using 166.00277 g/mol.
Worked examples
0.9% w/v potassium iodide 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 0.054216 mol/L, and it is 9 mg/mL if you are dosing by volume.
5% w/v potassium iodide in 100 mL
5% w/v is 5 g per 100 mL, so 100 mL needs 5 g. In molar terms that solution is 0.3012 mol/L, and it is 50 mg/mL if you are dosing by volume.
10% w/v potassium iodide in 1000 mL
10% w/v is 10 g per 100 mL, so 1000 mL needs 100 g. In molar terms that solution is 0.6024 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 iodide (KI)?
It is 166.00277 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 39.0983 (K) + 126.904 (I). One mole of potassium iodide therefore weighs 166.0028 g, and a gram of it is 6.024 mmol.
What percentage of potassium iodide is iodine?
76.45% by mass. Each formula unit contains 1 iodine atom contributing 126.904 g of the 166.0028 g total, so 100 g of potassium iodide contains 76.45 g of iodine and a kilogram contains 764.47 g of it.
How many grams of potassium iodide 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 iodide?
0.06024 mol/L, because 1% w/v is 10 g/L and 166.0028 g/L is one molar. A 0.9% solution is 0.054216 mol/L and a 5% solution is 0.3012 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 iodide that is also 60.24 mmol/L, which is the form a dose calculation usually wants.
Percentage strengths of potassium iodide
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Iodine (I) | 1 | 126.9045 | 126.9045 | 76.45% |
| Potassium (K) | 1 | 39.0983 | 39.0983 | 23.55% |
| Total — one mole of potassium iodide | 166.00277 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 166.00277 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 | 0.006024 |
| 0.25% w/v | 250 mg | 1.25 g | 2.5 g | 0.01506 |
| 0.5% w/v | 500 mg | 2.5 g | 5 g | 0.03012 |
| 0.9% w/v | 900 mg | 4.5 g | 9 g | 0.054216 |
| 1% w/v | 1 g | 5 g | 10 g | 0.06024 |
| 2% w/v | 2 g | 10 g | 20 g | 0.12048 |
| 3% w/v | 3 g | 15 g | 30 g | 0.18072 |
| 5% w/v | 5 g | 25 g | 50 g | 0.3012 |
| 10% w/v | 10 g | 50 g | 100 g | 0.6024 |
| 20% w/v | 20 g | 100 g | 200 g | 1.2048 |
A 1% w/v solution is 1 g in 100 mL, which for this compound is 0.06024 mol/L because 10 g/L divided by 166.00277 g/mol gives that figure.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Copper(II) sulfate | CuSO4 | 159.602 | 6.2656 |
| Iron(III) chloride | FeCl3 | 162.195 | 6.1654 |
| Trisodium phosphate | Na3PO4 | 163.9391 | 6.0998 |
| Potassium iodide (this page) | KI | 166.0028 | 6.024 |
| Silver nitrate | AgNO3 | 169.8722 | 5.8868 |
| Gypsum | CaSO4·2H2O | 172.164 | 5.8084 |
| Potassium sulfate | K2SO4 | 174.2526 | 5.7388 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.