Moles to grams for zinc sulfate heptahydrate is a multiplication by 287.541 g/mol. A mole weighs 287.541 g, 100 mmol weighs 28.7541 g, 10 mmol weighs 2.87541 g and a millimole is 287.541 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
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.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
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
n- The amount of zinc sulfate heptahydrate you need, in moles
287.541- The molar mass of zinc sulfate heptahydrate in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 287.541 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of zinc sulfate heptahydrate in grams
5 mmol × 287.541 g/mol = 1.43771 g, which is 1437.7 mg. Doubling the amount doubles the mass to 2.87541 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of zinc sulfate heptahydrate in grams
250 mmol × 287.541 g/mol = 71.8852 g, which is 71,885.2 mg. Doubling the amount doubles the mass to 143.77 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of zinc sulfate heptahydrate in grams
2 mol × 287.541 g/mol = 575.082 g, which is 575,082 mg. Doubling the amount doubles the mass to 1150.16 g, because the relationship is a plain proportion once the molar mass is fixed.
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 grams is one mole of zinc sulfate heptahydrate?
287.541 g, by definition of the molar mass. Two moles is 575.082 g and half a mole is 143.7705 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of zinc sulfate heptahydrate is 28.7541 g. One millimole is 287.541 mg and one micromole is 287.541 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 2.875 mg of zinc sulfate heptahydrate, weigh 28.75 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the hydrated form, ZnSO4·7H2O, at 287.541 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. A hydrate carries its water into the weighing, which is why the two figures are not interchangeable.
Moles to grams reference for 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000287541 | 0.287541 | 0.000293292 |
| 10 µmol | 0.00287541 | 2.87541 | 0.00293292 |
| 100 µmol | 0.0287541 | 28.7541 | 0.0293292 |
| 1 mmol | 0.287541 | 287.541 | 0.293292 |
| 10 mmol | 2.87541 | 2875.41 | 2.93292 |
| 50 mmol | 14.3771 | 14,377.1 | 14.6646 |
| 100 mmol | 28.7541 | 28,754.1 | 29.3292 |
| 250 mmol | 71.8852 | 71,885.2 | 73.323 |
| 500 mmol | 143.77 | 143,770 | 146.646 |
| 750 mmol | 215.656 | 215,656 | 219.969 |
| 1 mol | 287.541 | 287,541 | 293.292 |
| 2 mol | 575.082 | 575,082 | 586.584 |
| 5 mol | 1437.7 | 1437705.000000 | 1466.46 |
| 10 mol | 2875.41 | 2875410.000000 | 2932.92 |
Weigh is the amount multiplied by 287.541 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| 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.