Going from grams to moles for copper(II) sulfate pentahydrate means dividing by its molar mass, 249.677 g/mol. One gram is 0.00400517 mol, or 4.00517 mmol; 100 mg is 0.40052 mmol; and a 249.677 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, CuSO4·5H2O, which is where these figures come from.
The figure comes from the formula. Copper(II) sulfate pentahydrate is 63.546 (Cu) + 32.06 (S) + 9 × 15.999 (O) + 10 × 1.008 (H), which is 249.677 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: 9 of the 21 atoms in a formula unit are oxygen, and they account for 143.991 g of the 249.677 g, or 57.67% by mass. The derivation table below breaks the whole molecule down element by element.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.04005 mmol of copper(II) sulfate pentahydrate; on a three-decimal balance it is 1 mg, or 4.005 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats weighing.
The blue crystalline form — a fungicide, an electroplating salt and the one to weigh out unless a recipe says anhydrous. At 249.677 g/mol it is heavier than 28 of the 34 salts covered here, and that ranking matters more than it looks: sodium thiosulfate pentahydrate has a molar mass of 248.1715 g/mol, so a gram of it contains 0.607% more formula units than a gram of copper(II) sulfate pentahydrate. Weigh by mass and you are not weighing equal amounts of substance.
Watch which form you have. This page is the hydrate, CuSO4·5H2O, at 249.677 g/mol; the anhydrous salt is CuSO4 at 159.602 g/mol. The 90.075 g of water per mole is 36.08% of the weight, so weighing the hydrate as though it were anhydrous puts you 36.08% 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 you weighed, in grams
249.677- The molar mass of copper(II) sulfate pentahydrate in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 249.677 g/mol, the molar mass of copper(II) sulfate pentahydrate.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of copper(II) sulfate pentahydrate in moles
0.25 ÷ 249.677 = 0.00100129 mol, or 1.0013 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 4.005 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of copper(II) sulfate pentahydrate in moles
2 ÷ 249.677 = 0.00801035 mol, or 8.0103 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 4.005 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of copper(II) sulfate pentahydrate in moles
25 ÷ 249.677 = 0.100129 mol, or 100.13 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 4.005 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
How to read your score
Frequently asked questions
What is the molar mass of copper(II) sulfate pentahydrate (CuSO4·5H2O)?
It is 249.677 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 63.546 (Cu) + 32.06 (S) + 9 × 15.999 (O) + 10 × 1.008 (H). One mole of copper(II) sulfate pentahydrate therefore weighs 249.677 g, and a gram of it is 4.0052 mmol.
What percentage of copper(II) sulfate pentahydrate is oxygen?
57.67% by mass. Each formula unit contains 9 oxygen atoms contributing 143.991 g of the 249.677 g total, so 100 g of copper(II) sulfate pentahydrate contains 57.67 g of oxygen and a kilogram contains 576.71 g of it.
How do I convert grams of copper(II) sulfate pentahydrate to moles?
Divide the mass in grams by 249.677 g/mol. So 1 g is 0.00400517 mol, 10 g is 0.0400517 mol and 100 g is 0.400517 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of copper(II) sulfate pentahydrate?
0.400517 mol, which is 400.517 mmol. Read the other way, one mole of copper(II) sulfate pentahydrate is 249.677 g, so 100 g is 0.4005 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of copper(II) sulfate pentahydrate is 4.0052 µmol, 10 mg is 40.052 µmol and 100 mg is 0.40052 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of copper(II) sulfate pentahydrate, which is 0.00392507 mol rather than 0.00400517 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles reference for copper(II) sulfate pentahydrate
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 9 | 15.999 | 143.991 | 57.67% |
| Copper (Cu) | 1 | 63.546 | 63.546 | 25.45% |
| Sulfur (S) | 1 | 32.06 | 32.06 | 12.84% |
| Hydrogen (H) | 10 | 1.008 | 10.08 | 4.037% |
| Total — one mole of copper(II) sulfate pentahydrate | 249.677 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 249.677 g/mol.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000004 | 0.00400517 | 4.00517 |
| 5 mg | 0.000020 | 0.0200259 | 20.0259 |
| 10 mg | 0.000040 | 0.0400517 | 40.0517 |
| 25 mg | 0.000100129 | 0.100129 | 100.129 |
| 50 mg | 0.000200259 | 0.200259 | 200.259 |
| 100 mg | 0.000400517 | 0.400517 | 400.517 |
| 250 mg | 0.00100129 | 1.00129 | 1001.29 |
| 500 mg | 0.00200259 | 2.00259 | 2002.59 |
| 1 g | 0.00400517 | 4.00517 | 4005.17 |
| 2 g | 0.00801035 | 8.01035 | 8010.35 |
| 5 g | 0.0200259 | 20.0259 | 20,025.9 |
| 10 g | 0.0400517 | 40.0517 | 40,051.7 |
| 25 g | 0.100129 | 100.129 | 100,129 |
| 50 g | 0.200259 | 200.259 | 200,259 |
| 100 g | 0.400517 | 400.517 | 400,517 |
| 250 g | 1.00129 | 1001.29 | 1001293.671423 |
| 500 g | 2.00259 | 2002.59 | 2002587.342847 |
Every row is the mass divided by 249.677 g/mol. A balance reading to 1 mg resolves 4.005 µmol of this compound, which is the smallest step the middle columns can really move in.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Potassium sulfate | K2SO4 | 174.2526 | 5.7388 |
| Epsom salt | MgSO4·7H2O | 246.466 | 4.0574 |
| Sodium thiosulfate pentahydrate | Na2S2O3·5H2O | 248.1715 | 4.0295 |
| Copper(II) sulfate pentahydrate (this page) | 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 | ZnSO4·7H2O | 287.541 | 3.4778 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.