Going from grams to moles for potassium iodide means dividing by its molar mass, 166.00277 g/mol. One gram is 0.006024 mol, or 6.024 mmol; 100 mg is 0.6024 mmol; and a 166.0028 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, KI, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.06024 mmol of potassium iodide; on a three-decimal balance it is 1 mg, or 6.024 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats 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
m- The mass you weighed, in grams
166.00277- The molar mass of potassium iodide in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 166.00277 g/mol, the molar mass of potassium iodide.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of potassium iodide in moles
0.25 ÷ 166.0028 = 0.001506 mol, or 1.506 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.024 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of potassium iodide in moles
2 ÷ 166.0028 = 0.012048 mol, or 12.048 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.024 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of potassium iodide in moles
25 ÷ 166.0028 = 0.1506 mol, or 150.6 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.024 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
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 do I convert grams of potassium iodide to moles?
Divide the mass in grams by 166.00277 g/mol. So 1 g is 0.006024 mol, 10 g is 0.06024 mol and 100 g is 0.6024 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of potassium iodide?
0.6024 mol, which is 602.4 mmol. Read the other way, one mole of potassium iodide is 166.0028 g, so 100 g is 0.6024 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of potassium iodide is 6.024 µmol, 10 mg is 60.24 µmol and 100 mg is 0.6024 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of potassium iodide, which is 0.00590352 mol rather than 0.006024 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles reference 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000006 | 0.006024 | 6.024 |
| 5 mg | 0.000030 | 0.03012 | 30.12 |
| 10 mg | 0.000060 | 0.06024 | 60.24 |
| 25 mg | 0.0001506 | 0.1506 | 150.6 |
| 50 mg | 0.0003012 | 0.3012 | 301.2 |
| 100 mg | 0.0006024 | 0.6024 | 602.4 |
| 250 mg | 0.001506 | 1.506 | 1506 |
| 500 mg | 0.003012 | 3.012 | 3012 |
| 1 g | 0.006024 | 6.024 | 6024 |
| 2 g | 0.012048 | 12.048 | 12,048 |
| 5 g | 0.03012 | 30.12 | 30,120 |
| 10 g | 0.06024 | 60.24 | 60,240 |
| 25 g | 0.1506 | 150.6 | 150,600 |
| 50 g | 0.3012 | 301.2 | 301,200 |
| 100 g | 0.6024 | 602.4 | 602,400 |
| 250 g | 1.506 | 1506 | 1505998.966162 |
| 500 g | 3.012 | 3012 | 3011997.932324 |
Every row is the mass divided by 166.00277 g/mol. A balance reading to 1 mg resolves 6.024 µmol of this compound, which is the smallest step the middle columns can really move in.
| 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.