To prepare a solution of potassium iodide you need the grams, and grams is molarity × volume in litres × 166.00277 g/mol. A litre of 1 M takes 166.0028 g; 500 mL of 0.1 M takes 8.3001 g; 250 mL of 0.01 M takes 415.01 mg. Enter the strength you want and the volume you want it in, and the calculator returns the mass to weigh together with the same figure in milligrams and as a percentage strength.
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.
Order of operations matters more than precision here. Dissolve the solid in rather less than the final volume, then make up to the mark once it has all gone into solution and come back to room temperature — warm solutions read low when they cool. If the calculated mass is under about 20 mg, prepare ten times the strength and dilute it tenfold by pipette instead, because C₁V₁ = C₂V₂ carries less error than a marginal weighing.
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.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 3.3201 g of every 166.0028 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
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
166.00277- The molar mass of potassium iodide in g/mol, derived from its formula
m- The mass of potassium iodide to weigh out, in grams
How it works, step by step
- Enter the molarity you want, in mol/L.
- Enter the volume you want to make, in millilitres.
- The two are multiplied together and by 166.00277 g/mol.
- The result is the mass to weigh; dissolve it in part of the volume and then make up to the mark.
Worked examples
250 mL of 0.1 M potassium iodide
Weigh 4.15007 g. That is 0.1 M × 0.25 L × 166.0028 g/mol, and it comes to 4150.07 mg if your balance is set to milligrams. Dissolve in about 175 mL first, then make up to 250 mL.
500 mL of 0.5 M potassium iodide
Weigh 41.5007 g. That is 0.5 M × 0.5 L × 166.0028 g/mol, and it comes to 41,500.7 mg if your balance is set to milligrams. Dissolve in about 350 mL first, then make up to 500 mL.
1000 mL of 1 M potassium iodide
Weigh 166.003 g. That is 1 M × 1 L × 166.0028 g/mol, and it comes to 166,003 mg if your balance is set to milligrams. Dissolve in about 700 mL first, then make up to 1000 mL.
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 much potassium iodide do I need for 1 litre of 1 M solution?
166.00277 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 16.6003 g, and for 1 L of 0.01 M it is 1.66003 g.
How much for 100 mL of 0.1 M?
1.66003 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 166.0028 g. In milligrams that is 1660.03 mg.
Do I dissolve the solid first or make up the volume first?
Dissolve first, in perhaps 70% of the final volume, then make up to the mark. Adding solid to a full flask overshoots the volume and leaves the solution weak, and undissolved solid at the mark means the strength keeps changing as it goes in.
Can I dilute a stock solution instead of weighing?
Yes, and it is more accurate for small amounts. C₁V₁ = C₂V₂: to get 500 mL of 0.01 M from a 0.5 M stock, take 10 mL of stock and make up to 500 mL. Weighing the 830.01 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes for 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.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 16.6003 mg | 41.5007 mg | 83.0014 mg | 166.003 mg |
| 0.005 M | 83.0014 mg | 207.503 mg | 415.007 mg | 830.014 mg |
| 0.01 M | 166.003 mg | 415.007 mg | 830.014 mg | 1.66003 g |
| 0.05 M | 830.014 mg | 2.07503 g | 4.15007 g | 8.30014 g |
| 0.1 M | 1.66003 g | 4.15007 g | 8.30014 g | 16.6003 g |
| 0.15 M | 2.49004 g | 6.2251 g | 12.4502 g | 24.9004 g |
| 0.2 M | 3.32006 g | 8.30014 g | 16.6003 g | 33.2006 g |
| 0.25 M | 4.15007 g | 10.3752 g | 20.7503 g | 41.5007 g |
| 0.5 M | 8.30014 g | 20.7503 g | 41.5007 g | 83.0014 g |
| 1 M | 16.6003 g | 41.5007 g | 83.0014 g | 166.003 g |
| 2 M | 33.2006 g | 83.0014 g | 166.003 g | 332.006 g |
Each cell is molarity × volume in litres × 166.00277 g/mol. Dissolve the solid first and then make up to the marked volume — adding solid to a full volume overshoots it.
| 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.