The molar mass of zinc sulfate heptahydrate, ZnSO4·7H2O, is 287.541 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 287.541 g, and a gram of it is 2.094e+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. Zinc sulfate heptahydrate is 65.38 (Zn) + 32.06 (S) + 11 × 15.999 (O) + 14 × 1.008 (H), which is 287.541 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 287.541 g, or 61.2% 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 zinc sulfate heptahydrate is 287.541 mg and one micromole is 287.541 µg, so a balance reading to a milligram places you within 3.478 µmol. A single formula unit weighs 4.7747e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
The zinc source in fertiliser and in supplements, and an astringent. The monohydrate at 179.47 g per mole is also sold, so check which you have. At 287.541 g/mol it is heavier than 31 of the 34 salts covered here, and that ranking matters more than it looks: iron(II) sulfate heptahydrate has a molar mass of 278.006 g/mol, so a gram of it contains 3.43% more formula units than a gram of zinc sulfate heptahydrate. Weigh by mass and you are not weighing equal amounts of substance.
Watch which form you have. This page is the hydrate, ZnSO4·7H2O, at 287.541 g/mol; the anhydrous salt is ZnSO4 at 161.436 g/mol. The 126.105 g of water per mole is 43.86% of the weight, so weighing the hydrate as though it were anhydrous puts you 43.86% 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 zinc sulfate heptahydrate in grams
287.541- The molar mass of zinc 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 zinc sulfate heptahydrate in grams.
- It is divided by the molar mass, 287.541 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 zinc sulfate heptahydrate as a molecule count
0.5 g divided by 287.541 g/mol is 0.00173888 mol, and multiplying by the Avogadro constant gives 1.047e+21 formula units of zinc sulfate heptahydrate. The same mass is 1.7389 mmol, which is the figure a reaction is actually planned in.
5 g of zinc sulfate heptahydrate as a molecule count
5 g divided by 287.541 g/mol is 0.0173888 mol, and multiplying by the Avogadro constant gives 1.047e+22 formula units of zinc sulfate heptahydrate. The same mass is 17.389 mmol, which is the figure a reaction is actually planned in.
50 g of zinc sulfate heptahydrate as a molecule count
50 g divided by 287.541 g/mol is 0.173888 mol, and multiplying by the Avogadro constant gives 1.047e+23 formula units of zinc sulfate heptahydrate. The same mass is 173.89 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 zinc sulfate heptahydrate (ZnSO4·7H2O)?
It is 287.541 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 65.38 (Zn) + 32.06 (S) + 11 × 15.999 (O) + 14 × 1.008 (H). One mole of zinc sulfate heptahydrate therefore weighs 287.541 g, and a gram of it is 3.4778 mmol.
What percentage of zinc sulfate heptahydrate is oxygen?
61.2% by mass. Each formula unit contains 11 oxygen atoms contributing 175.989 g of the 287.541 g total, so 100 g of zinc sulfate heptahydrate contains 61.2 g of oxygen and a kilogram contains 612.05 g of it.
How many molecules are in one gram of zinc sulfate heptahydrate?
About 2.094e+21. One gram is 0.00347776 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 287.541.
How much does a single molecule of zinc sulfate heptahydrate weigh?
4.7747e-22 g, which is 287.541 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 zinc sulfate heptahydrate it is 287.541. Molar mass carries grams per mole: 287.541 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 zinc sulfate heptahydrate comes out at 287.541 g/mol rather than 288.
Molar mass, composition and mole equivalents of zinc sulfate heptahydrate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 11 | 15.999 | 175.989 | 61.2% |
| Zinc (Zn) | 1 | 65.38 | 65.38 | 22.74% |
| Sulfur (S) | 1 | 32.06 | 32.06 | 11.15% |
| Hydrogen (H) | 14 | 1.008 | 14.112 | 4.908% |
| Total — one mole of zinc sulfate heptahydrate | 287.541 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 287.541 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000287541 | 0.287541 | 6.022e+17 |
| 10 µmol | 0.00287541 | 2.87541 | 6.022e+18 |
| 100 µmol | 0.0287541 | 28.7541 | 6.022e+19 |
| 1 mmol | 0.287541 | 287.541 | 6.022e+20 |
| 10 mmol | 2.87541 | 2875.41 | 6.022e+21 |
| 50 mmol | 14.3771 | 14,377.1 | 3.011e+22 |
| 100 mmol | 28.7541 | 28,754.1 | 6.022e+22 |
| 250 mmol | 71.8852 | 71,885.2 | 1.506e+23 |
| 500 mmol | 143.77 | 143,770 | 3.011e+23 |
| 750 mmol | 215.656 | 215,656 | 4.517e+23 |
| 1 mol | 287.541 | 287,541 | 6.022e+23 |
| 2 mol | 575.082 | 575,082 | 1.204e+24 |
| 5 mol | 1437.7 | 1437705.000000 | 3.011e+24 |
| 10 mol | 2875.41 | 2875410.000000 | 6.022e+24 |
Mass is the amount multiplied by 287.541 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 | FeSO4·7H2O | 278.006 | 3.597 |
| Zinc sulfate heptahydrate (this page) | 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.