The molar mass of Epsom salt, MgSO4·7H2O, is 246.466 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 246.466 g, and a gram of it is 2.443e+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. Epsom salt is 24.305 (Mg) + 32.06 (S) + 11 × 15.999 (O) + 14 × 1.008 (H), which is 246.466 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 246.466 g, or 71.4% 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 Epsom salt is 246.466 mg and one micromole is 246.466 µg, so a balance reading to a milligram places you within 4.057 µmol. A single formula unit weighs 4.0927e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Magnesium sulfate heptahydrate: bath salts, a magnesium source for plants, and a laboratory standard. Seven waters make it twice the weight per mole. At 246.466 g/mol it is heavier than 26 of the 34 salts covered here, and that ranking matters more than it looks: potassium sulfate has a molar mass of 174.2526 g/mol, so a gram of it contains 41.4% more formula units than a gram of Epsom salt. Weigh by mass and you are not weighing equal amounts of substance.
Watch which form you have. This page is the hydrate, MgSO4·7H2O, at 246.466 g/mol; the anhydrous salt is MgSO4 at 120.361 g/mol. The 126.105 g of water per mole is 51.17% of the weight, so weighing the hydrate as though it were anhydrous puts you 51.17% 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 Epsom salt in grams
246.466- The molar mass of Epsom salt 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 Epsom salt in grams.
- It is divided by the molar mass, 246.466 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 Epsom salt as a molecule count
0.5 g divided by 246.466 g/mol is 0.00202868 mol, and multiplying by the Avogadro constant gives 1.222e+21 formula units of Epsom salt. The same mass is 2.0287 mmol, which is the figure a reaction is actually planned in.
5 g of Epsom salt as a molecule count
5 g divided by 246.466 g/mol is 0.0202868 mol, and multiplying by the Avogadro constant gives 1.222e+22 formula units of Epsom salt. The same mass is 20.287 mmol, which is the figure a reaction is actually planned in.
50 g of Epsom salt as a molecule count
50 g divided by 246.466 g/mol is 0.202868 mol, and multiplying by the Avogadro constant gives 1.222e+23 formula units of Epsom salt. The same mass is 202.87 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 Epsom salt (MgSO4·7H2O)?
It is 246.466 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 24.305 (Mg) + 32.06 (S) + 11 × 15.999 (O) + 14 × 1.008 (H). One mole of Epsom salt therefore weighs 246.466 g, and a gram of it is 4.0574 mmol.
What percentage of Epsom salt is oxygen?
71.4% by mass. Each formula unit contains 11 oxygen atoms contributing 175.989 g of the 246.466 g total, so 100 g of Epsom salt contains 71.4 g of oxygen and a kilogram contains 714.05 g of it.
How many molecules are in one gram of Epsom salt?
About 2.443e+21. One gram is 0.00405735 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 246.466.
How much does a single molecule of Epsom salt weigh?
4.0927e-22 g, which is 246.466 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 Epsom salt it is 246.466. Molar mass carries grams per mole: 246.466 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 Epsom salt comes out at 246.466 g/mol rather than 246.
Molar mass, composition and mole equivalents of Epsom salt
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 11 | 15.999 | 175.989 | 71.4% |
| Sulfur (S) | 1 | 32.06 | 32.06 | 13.01% |
| Magnesium (Mg) | 1 | 24.305 | 24.305 | 9.861% |
| Hydrogen (H) | 14 | 1.008 | 14.112 | 5.726% |
| Total — one mole of Epsom salt | 246.466 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 246.466 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000246466 | 0.246466 | 6.022e+17 |
| 10 µmol | 0.00246466 | 2.46466 | 6.022e+18 |
| 100 µmol | 0.0246466 | 24.6466 | 6.022e+19 |
| 1 mmol | 0.246466 | 246.466 | 6.022e+20 |
| 10 mmol | 2.46466 | 2464.66 | 6.022e+21 |
| 50 mmol | 12.3233 | 12,323.3 | 3.011e+22 |
| 100 mmol | 24.6466 | 24,646.6 | 6.022e+22 |
| 250 mmol | 61.6165 | 61,616.5 | 1.506e+23 |
| 500 mmol | 123.233 | 123,233 | 3.011e+23 |
| 750 mmol | 184.85 | 184,850 | 4.517e+23 |
| 1 mol | 246.466 | 246,466 | 6.022e+23 |
| 2 mol | 492.932 | 492,932 | 1.204e+24 |
| 5 mol | 1232.33 | 1232330.000000 | 3.011e+24 |
| 10 mol | 2464.66 | 2464660.000000 | 6.022e+24 |
Mass is the amount multiplied by 246.466 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 |
|---|---|---|---|
| Silver nitrate | AgNO3 | 169.8722 | 5.8868 |
| Gypsum | CaSO4·2H2O | 172.164 | 5.8084 |
| Potassium sulfate | K2SO4 | 174.2526 | 5.7388 |
| Epsom salt (this page) | MgSO4·7H2O | 246.466 | 4.0574 |
| 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 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.