Moles to grams for potassium iodide is a multiplication by 166.00277 g/mol. A mole weighs 166.0028 g, 100 mmol weighs 16.6003 g, 10 mmol weighs 1.66003 g and a millimole is 166.003 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
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.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
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
n- The amount of potassium iodide you need, in moles
166.00277- The molar mass of potassium iodide in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 166.00277 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of potassium iodide in grams
5 mmol × 166.0028 g/mol = 0.830014 g, which is 830.014 mg. Doubling the amount doubles the mass to 1.66003 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of potassium iodide in grams
250 mmol × 166.0028 g/mol = 41.5007 g, which is 41,500.7 mg. Doubling the amount doubles the mass to 83.0014 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of potassium iodide in grams
2 mol × 166.0028 g/mol = 332.006 g, which is 332,006 mg. Doubling the amount doubles the mass to 664.011 g, because the relationship is a plain proportion once the molar mass is fixed.
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 is one mole of potassium iodide?
166.00277 g, by definition of the molar mass. Two moles is 332.0055 g and half a mole is 83.00138 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of potassium iodide is 16.6003 g. One millimole is 166.003 mg and one micromole is 166.003 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 1.66 mg of potassium iodide, weigh 16.6 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the anhydrous form, KI, at 166.00277 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. Salts that are sold as hydrates weigh more per mole because the water is part of the crystal.
Moles to grams 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000166003 | 0.166003 | 0.000169323 |
| 10 µmol | 0.00166003 | 1.66003 | 0.00169323 |
| 100 µmol | 0.0166003 | 16.6003 | 0.0169323 |
| 1 mmol | 0.166003 | 166.003 | 0.169323 |
| 10 mmol | 1.66003 | 1660.03 | 1.69323 |
| 50 mmol | 8.30014 | 8300.14 | 8.46614 |
| 100 mmol | 16.6003 | 16,600.3 | 16.9323 |
| 250 mmol | 41.5007 | 41,500.7 | 42.3307 |
| 500 mmol | 83.0014 | 83,001.4 | 84.6614 |
| 750 mmol | 124.502 | 124,502 | 126.992 |
| 1 mol | 166.003 | 166,003 | 169.323 |
| 2 mol | 332.006 | 332,006 | 338.646 |
| 5 mol | 830.014 | 830,014 | 846.614 |
| 10 mol | 1660.03 | 1660027.700000 | 1693.23 |
Weigh is the amount multiplied by 166.00277 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| 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.