The molar mass of iron(II) sulfate heptahydrate, FeSO4·7H2O, is 278.006 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 278.006 g, and a gram of it is 2.166e+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(II) sulfate heptahydrate is 55.845 (Fe) + 32.06 (S) + 11 × 15.999 (O) + 14 × 1.008 (H), which is 278.006 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 11 of the 27 atoms in a formula unit are oxygen, and they account for 175.989 g of the 278.006 g, or 63.3% 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(II) sulfate heptahydrate is 278.006 mg and one micromole is 278.006 µg, so a balance reading to a milligram places you within 3.597 µmol. A single formula unit weighs 4.6164e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Green vitriol: an iron supplement, a lawn moss killer and a titration standard. It oxidises in air, so old crystals read low in iron(II). At 278.006 g/mol it is heavier than 30 of the 34 salts covered here, and that ranking matters more than it looks: sodium citrate has a molar mass of 258.0683 g/mol, so a gram of it contains 7.73% more formula units than a gram of iron(II) sulfate heptahydrate. Weigh by mass and you are not weighing equal amounts of substance.
Watch which form you have. This page is the hydrate, FeSO4·7H2O, at 278.006 g/mol; the anhydrous salt is FeSO4 at 151.901 g/mol. The 126.105 g of water per mole is 45.36% of the weight, so weighing the hydrate as though it were anhydrous puts you 45.36% short on the compound itself. Suppliers sell both, and the label is the only way to tell them apart by eye.
The formula
m- The mass of iron(II) sulfate heptahydrate in grams
278.006- The molar mass of iron(II) sulfate heptahydrate 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(II) sulfate heptahydrate in grams.
- It is divided by the molar mass, 278.006 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(II) sulfate heptahydrate as a molecule count
0.5 g divided by 278.006 g/mol is 0.00179852 mol, and multiplying by the Avogadro constant gives 1.083e+21 formula units of iron(II) sulfate heptahydrate. The same mass is 1.7985 mmol, which is the figure a reaction is actually planned in.
5 g of iron(II) sulfate heptahydrate as a molecule count
5 g divided by 278.006 g/mol is 0.0179852 mol, and multiplying by the Avogadro constant gives 1.083e+22 formula units of iron(II) sulfate heptahydrate. The same mass is 17.985 mmol, which is the figure a reaction is actually planned in.
50 g of iron(II) sulfate heptahydrate as a molecule count
50 g divided by 278.006 g/mol is 0.179852 mol, and multiplying by the Avogadro constant gives 1.083e+23 formula units of iron(II) sulfate heptahydrate. The same mass is 179.85 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 iron(II) sulfate heptahydrate (FeSO4·7H2O)?
It is 278.006 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 55.845 (Fe) + 32.06 (S) + 11 × 15.999 (O) + 14 × 1.008 (H). One mole of iron(II) sulfate heptahydrate therefore weighs 278.006 g, and a gram of it is 3.597 mmol.
What percentage of iron(II) sulfate heptahydrate is oxygen?
63.3% by mass. Each formula unit contains 11 oxygen atoms contributing 175.989 g of the 278.006 g total, so 100 g of iron(II) sulfate heptahydrate contains 63.3 g of oxygen and a kilogram contains 633.04 g of it.
How many molecules are in one gram of iron(II) sulfate heptahydrate?
About 2.166e+21. One gram is 0.00359704 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 278.006.
How much does a single molecule of iron(II) sulfate heptahydrate weigh?
4.6164e-22 g, which is 278.006 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(II) sulfate heptahydrate it is 278.006. Molar mass carries grams per mole: 278.006 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. Oxygen is quoted as 15.999 rather than a whole number for that reason, and the same applies to the other elements here, which is why iron(II) sulfate heptahydrate comes out at 278.006 g/mol rather than 278.
Molar mass, composition and mole equivalents of iron(II) sulfate heptahydrate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 11 | 15.999 | 175.989 | 63.3% |
| Iron (Fe) | 1 | 55.845 | 55.845 | 20.09% |
| Sulfur (S) | 1 | 32.06 | 32.06 | 11.53% |
| Hydrogen (H) | 14 | 1.008 | 14.112 | 5.076% |
| Total — one mole of iron(II) sulfate heptahydrate | 278.006 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 278.006 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000278006 | 0.278006 | 6.022e+17 |
| 10 µmol | 0.00278006 | 2.78006 | 6.022e+18 |
| 100 µmol | 0.0278006 | 27.8006 | 6.022e+19 |
| 1 mmol | 0.278006 | 278.006 | 6.022e+20 |
| 10 mmol | 2.78006 | 2780.06 | 6.022e+21 |
| 50 mmol | 13.9003 | 13,900.3 | 3.011e+22 |
| 100 mmol | 27.8006 | 27,800.6 | 6.022e+22 |
| 250 mmol | 69.5015 | 69,501.5 | 1.506e+23 |
| 500 mmol | 139.003 | 139,003 | 3.011e+23 |
| 750 mmol | 208.505 | 208,504 | 4.517e+23 |
| 1 mol | 278.006 | 278,006 | 6.022e+23 |
| 2 mol | 556.012 | 556,012 | 1.204e+24 |
| 5 mol | 1390.03 | 1390030.000000 | 3.011e+24 |
| 10 mol | 2780.06 | 2780060.000000 | 6.022e+24 |
Mass is the amount multiplied by 278.006 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 thiosulfate pentahydrate | Na2S2O3·5H2O | 248.1715 | 4.0295 |
| Copper(II) sulfate pentahydrate | CuSO4·5H2O | 249.677 | 4.0052 |
| Sodium citrate | Na3C6H5O7 | 258.0683 | 3.8749 |
| Iron(II) sulfate heptahydrate (this page) | FeSO4·7H2O | 278.006 | 3.597 |
| Zinc sulfate heptahydrate | ZnSO4·7H2O | 287.541 | 3.4778 |
| Potassium dichromate | K2Cr2O7 | 294.1818 | 3.3993 |
| Aluminium sulfate | Al2(SO4)3 | 342.1311 | 2.9229 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.