Molar MassMolar mass of potassium iodide (KI)
The molar mass of potassium iodide, KI, is 166.00 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 166.00 grams.
About potassium iodide
Potassium iodide is a white salt added to table salt to supply iodine. In the laboratory it is used in iodine titrations and it gives a bright yellow precipitate with lead nitrate.
The working
The formula KI has 1 potassium atom and 1 iodine atom, 2 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Potassium (K) | 1 | 39.098 | 39.098 | 23.55% |
| Iodine (I) | 1 | 126.90 | 126.900 | 76.45% |
39.098 + 126.90 = 165.998 g/mol
With the rounded atomic masses used in many school books (K = 39, I = 127) the answer is 166 g/mol. The difference from 166.00 is too small to matter in most school sums.
Percentage composition
By mass, KI is 23.55% potassium and 76.45% iodine. The largest share comes from iodine. In a 100 g sample there are 23.6 g of potassium and 76.4 g of iodine.
Worked examples
Grams to moles. How many moles are in 50 g of KI? Divide the mass by the molar mass: 50 ÷ 166.00 = 0.3012 mol.
Moles to grams. What is the mass of 0.4 mol of KI? Multiply: 0.4 × 166.00 = 66.4 g.
Counting particles. 50 g contains 0.3012 mol × 6.022 × 1023 = 1.814 × 1023 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 166.00 = 16.6 g of KI in each litre. For 250 mL, weigh 4.15 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 16.6 g |
| 0.5 mol | 83 g |
| 1 mol | 166 g |
| 2 mol | 332 g |
| 5 mol | 830 g |
| Mass | Moles |
|---|---|
| 1 g | 0.006024 mol |
| 10 g | 0.06024 mol |
| 100 g | 0.6024 mol |
| 1 kg | 6.024 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of KI?
The molar mass of potassium iodide (KI) is 166.00 g/mol, or 165.998 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of KI?
100 ÷ 166.00 = 0.6024 mol.
What is the mass of one formula unit of KI?
Divide the molar mass by the Avogadro constant: 166.00 ÷ 6.022 × 1023 = 2.756 × 10-22 g.
What is the percentage of iodine in KI?
Iodine makes up 76.45% of the mass of KI: 1 × 126.90 ÷ 165.998 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.