The molar mass of copper(II) sulfate pentahydrate, CuSO4·5H2O, is 249.677 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 249.677 g, and a gram of it is 2.412e+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. 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.
The numbers a bench actually uses sit at the millimole scale. One millimole of copper(II) sulfate pentahydrate is 249.677 mg and one micromole is 249.677 µg, so a balance reading to a milligram places you within 4.005 µmol. A single formula unit weighs 4.1460e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
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 of copper(II) sulfate pentahydrate in grams
249.677- The molar mass of copper(II) sulfate pentahydrate 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 copper(II) sulfate pentahydrate in grams.
- It is divided by the molar mass, 249.677 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 copper(II) sulfate pentahydrate as a molecule count
0.5 g divided by 249.677 g/mol is 0.00200259 mol, and multiplying by the Avogadro constant gives 1.206e+21 formula units of copper(II) sulfate pentahydrate. The same mass is 2.0026 mmol, which is the figure a reaction is actually planned in.
5 g of copper(II) sulfate pentahydrate as a molecule count
5 g divided by 249.677 g/mol is 0.0200259 mol, and multiplying by the Avogadro constant gives 1.206e+22 formula units of copper(II) sulfate pentahydrate. The same mass is 20.026 mmol, which is the figure a reaction is actually planned in.
50 g of copper(II) sulfate pentahydrate as a molecule count
50 g divided by 249.677 g/mol is 0.200259 mol, and multiplying by the Avogadro constant gives 1.206e+23 formula units of copper(II) sulfate pentahydrate. The same mass is 200.26 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 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 many molecules are in one gram of copper(II) sulfate pentahydrate?
About 2.412e+21. One gram is 0.00400517 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 249.677.
How much does a single molecule of copper(II) sulfate pentahydrate weigh?
4.1460e-22 g, which is 249.677 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 copper(II) sulfate pentahydrate it is 249.677. Molar mass carries grams per mole: 249.677 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 copper(II) sulfate pentahydrate comes out at 249.677 g/mol rather than 250.
Molar mass, composition and mole equivalents of 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.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000249677 | 0.249677 | 6.022e+17 |
| 10 µmol | 0.00249677 | 2.49677 | 6.022e+18 |
| 100 µmol | 0.0249677 | 24.9677 | 6.022e+19 |
| 1 mmol | 0.249677 | 249.677 | 6.022e+20 |
| 10 mmol | 2.49677 | 2496.77 | 6.022e+21 |
| 50 mmol | 12.4839 | 12,483.9 | 3.011e+22 |
| 100 mmol | 24.9677 | 24,967.7 | 6.022e+22 |
| 250 mmol | 62.4193 | 62,419.3 | 1.506e+23 |
| 500 mmol | 124.839 | 124,839 | 3.011e+23 |
| 750 mmol | 187.258 | 187,258 | 4.517e+23 |
| 1 mol | 249.677 | 249,677 | 6.022e+23 |
| 2 mol | 499.354 | 499,354 | 1.204e+24 |
| 5 mol | 1248.39 | 1248385.000000 | 3.011e+24 |
| 10 mol | 2496.77 | 2496770.000000 | 6.022e+24 |
Mass is the amount multiplied by 249.677 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 |
|---|---|---|---|
| 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.