The molar mass of potassium iodide, KI, is 166.00277 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 166.0028 g, and a gram of it is 3.628e+21 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside 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.
The numbers a bench actually uses sit at the millimole scale. One millimole of potassium iodide is 166.003 mg and one micromole is 166.003 µg, so a balance reading to a milligram places you within 6.024 µmol. A single formula unit weighs 2.7565e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
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
m- The mass of potassium iodide in grams
166.00277- The molar mass of potassium iodide in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of potassium iodide in grams.
- It is divided by the molar mass, 166.00277 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of potassium iodide as a molecule count
0.5 g divided by 166.0028 g/mol is 0.003012 mol, and multiplying by the Avogadro constant gives 1.814e+21 formula units of potassium iodide. The same mass is 3.012 mmol, which is the figure a reaction is actually planned in.
5 g of potassium iodide as a molecule count
5 g divided by 166.0028 g/mol is 0.03012 mol, and multiplying by the Avogadro constant gives 1.814e+22 formula units of potassium iodide. The same mass is 30.12 mmol, which is the figure a reaction is actually planned in.
50 g of potassium iodide as a molecule count
50 g divided by 166.0028 g/mol is 0.3012 mol, and multiplying by the Avogadro constant gives 1.814e+23 formula units of potassium iodide. The same mass is 301.2 mmol, which is the figure a reaction is actually planned in.
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 molecules are in one gram of potassium iodide?
About 3.628e+21. One gram is 0.006024 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 166.0028.
How much does a single molecule of potassium iodide weigh?
2.7565e-22 g, which is 166.00277 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for potassium iodide it is 166.00277. Molar mass carries grams per mole: 166.00277 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Iodine is quoted as 126.9045 rather than a whole number for that reason, and the same applies to the other elements here, which is why potassium iodide comes out at 166.00277 g/mol rather than 166.
Molar mass, composition and mole equivalents 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.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000166003 | 0.166003 | 6.022e+17 |
| 10 µmol | 0.00166003 | 1.66003 | 6.022e+18 |
| 100 µmol | 0.0166003 | 16.6003 | 6.022e+19 |
| 1 mmol | 0.166003 | 166.003 | 6.022e+20 |
| 10 mmol | 1.66003 | 1660.03 | 6.022e+21 |
| 50 mmol | 8.30014 | 8300.14 | 3.011e+22 |
| 100 mmol | 16.6003 | 16,600.3 | 6.022e+22 |
| 250 mmol | 41.5007 | 41,500.7 | 1.506e+23 |
| 500 mmol | 83.0014 | 83,001.4 | 3.011e+23 |
| 750 mmol | 124.502 | 124,502 | 4.517e+23 |
| 1 mol | 166.003 | 166,003 | 6.022e+23 |
| 2 mol | 332.006 | 332,006 | 1.204e+24 |
| 5 mol | 830.014 | 830,014 | 3.011e+24 |
| 10 mol | 1660.03 | 1660027.700000 | 6.022e+24 |
Mass is the amount multiplied by 166.00277 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| 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.