The molar mass of iron(III) chloride, FeCl3, is 162.195 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 162.195 g, and a gram of it is 3.713e+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. Iron(III) chloride is 55.845 (Fe) + 3 × 35.45 (Cl), which is 162.195 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Chlorine makes up the largest share of the mass: 3 of the 4 atoms in a formula unit are chlorine, and they account for 106.35 g of the 162.195 g, or 65.57% 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 iron(III) chloride is 162.195 mg and one micromole is 162.195 µg, so a balance reading to a milligram places you within 6.165 µmol. A single formula unit weighs 2.6933e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
A water treatment coagulant and the etchant for copper circuit boards. It fumes and stains, and its solutions are strongly acidic. At 162.195 g/mol it is heavier than 20 of the 34 salts covered here, and that ranking matters more than it looks: copper(II) sulfate has a molar mass of 159.602 g/mol, so a gram of it contains 1.62% more formula units than a gram of iron(III) chloride. 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 3.2439 g of a nominal 162.195 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 iron(III) chloride in grams
162.195- The molar mass of iron(III) chloride 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 iron(III) chloride in grams.
- It is divided by the molar mass, 162.195 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 iron(III) chloride as a molecule count
0.5 g divided by 162.195 g/mol is 0.00308271 mol, and multiplying by the Avogadro constant gives 1.856e+21 formula units of iron(III) chloride. The same mass is 3.0827 mmol, which is the figure a reaction is actually planned in.
5 g of iron(III) chloride as a molecule count
5 g divided by 162.195 g/mol is 0.0308271 mol, and multiplying by the Avogadro constant gives 1.856e+22 formula units of iron(III) chloride. The same mass is 30.827 mmol, which is the figure a reaction is actually planned in.
50 g of iron(III) chloride as a molecule count
50 g divided by 162.195 g/mol is 0.308271 mol, and multiplying by the Avogadro constant gives 1.856e+23 formula units of iron(III) chloride. The same mass is 308.27 mmol, which is the figure a reaction is actually planned in.
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Frequently asked questions
What is the molar mass of iron(III) chloride (FeCl3)?
It is 162.195 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 55.845 (Fe) + 3 × 35.45 (Cl). One mole of iron(III) chloride therefore weighs 162.195 g, and a gram of it is 6.1654 mmol.
What percentage of iron(III) chloride is chlorine?
65.57% by mass. Each formula unit contains 3 chlorine atoms contributing 106.35 g of the 162.195 g total, so 100 g of iron(III) chloride contains 65.57 g of chlorine and a kilogram contains 655.69 g of it.
How many molecules are in one gram of iron(III) chloride?
About 3.713e+21. One gram is 0.00616542 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 162.195.
How much does a single molecule of iron(III) chloride weigh?
2.6933e-22 g, which is 162.195 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 iron(III) chloride it is 162.195. Molar mass carries grams per mole: 162.195 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. Chlorine is quoted as 35.45 rather than a whole number for that reason, and the same applies to the other elements here, which is why iron(III) chloride comes out at 162.195 g/mol rather than 162.
Molar mass, composition and mole equivalents of iron(III) chloride
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Chlorine (Cl) | 3 | 35.45 | 106.35 | 65.57% |
| Iron (Fe) | 1 | 55.845 | 55.845 | 34.43% |
| Total — one mole of iron(III) chloride | 162.195 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 162.195 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000162195 | 0.162195 | 6.022e+17 |
| 10 µmol | 0.00162195 | 1.62195 | 6.022e+18 |
| 100 µmol | 0.0162195 | 16.2195 | 6.022e+19 |
| 1 mmol | 0.162195 | 162.195 | 6.022e+20 |
| 10 mmol | 1.62195 | 1621.95 | 6.022e+21 |
| 50 mmol | 8.10975 | 8109.75 | 3.011e+22 |
| 100 mmol | 16.2195 | 16,219.5 | 6.022e+22 |
| 250 mmol | 40.5487 | 40,548.8 | 1.506e+23 |
| 500 mmol | 81.0975 | 81,097.5 | 3.011e+23 |
| 750 mmol | 121.646 | 121,646 | 4.517e+23 |
| 1 mol | 162.195 | 162,195 | 6.022e+23 |
| 2 mol | 324.39 | 324,390 | 1.204e+24 |
| 5 mol | 810.975 | 810,975 | 3.011e+24 |
| 10 mol | 1621.95 | 1621950.000000 | 6.022e+24 |
Mass is the amount multiplied by 162.195 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 |
|---|---|---|---|
| Sodium sulfate | Na2SO4 | 142.0355 | 7.0405 |
| Potassium permanganate | KMnO4 | 158.0323 | 6.3278 |
| Copper(II) sulfate | CuSO4 | 159.602 | 6.2656 |
| Iron(III) chloride (this page) | FeCl3 | 162.195 | 6.1654 |
| Trisodium phosphate | Na3PO4 | 163.9391 | 6.0998 |
| Potassium iodide | KI | 166.0028 | 6.024 |
| Silver nitrate | AgNO3 | 169.8722 | 5.8868 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.