The molar mass of zinc oxide, ZnO, is 81.379 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 81.379 g, and a gram of it is 7.4e+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 oxide is 65.38 (Zn) + 15.999 (O), which is 81.379 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Zinc makes up the largest share of the mass: 1 of the 2 atoms in a formula unit is zinc, and they account for 65.38 g of the 81.379 g, or 80.34% 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 oxide is 81.379 mg and one micromole is 81.379 µg, so a balance reading to a milligram places you within 12.29 µmol. A single formula unit weighs 1.3513e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Nappy cream, mineral sunscreen and the accelerator in rubber vulcanisation. It blocks ultraviolet without dissolving, so it sits on the skin. At 81.379 g/mol it is heavier than 4 of the 8 oxides covered here, and that ranking matters more than it looks: titanium dioxide has a molar mass of 79.865 g/mol, so a gram of it contains 1.9% more formula units than a gram of zinc oxide. Weigh by mass and you are not weighing equal amounts of substance.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 1.6276 g of every 81.379 g is not the compound — and any water picked up from the air does the same. Both are systematic, so they shift every solution made from that jar in the same direction.
The formula
m- The mass of zinc oxide in grams
81.379- The molar mass of zinc oxide 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 oxide in grams.
- It is divided by the molar mass, 81.379 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 oxide as a molecule count
0.5 g divided by 81.379 g/mol is 0.00614409 mol, and multiplying by the Avogadro constant gives 3.7e+21 formula units of zinc oxide. The same mass is 6.1441 mmol, which is the figure a reaction is actually planned in.
5 g of zinc oxide as a molecule count
5 g divided by 81.379 g/mol is 0.0614409 mol, and multiplying by the Avogadro constant gives 3.7e+22 formula units of zinc oxide. The same mass is 61.441 mmol, which is the figure a reaction is actually planned in.
50 g of zinc oxide as a molecule count
50 g divided by 81.379 g/mol is 0.614409 mol, and multiplying by the Avogadro constant gives 3.7e+23 formula units of zinc oxide. The same mass is 614.41 mmol, which is the figure a reaction is actually planned in.
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Frequently asked questions
What is the molar mass of zinc oxide (ZnO)?
It is 81.379 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 65.38 (Zn) + 15.999 (O). One mole of zinc oxide therefore weighs 81.379 g, and a gram of it is 12.288 mmol.
What percentage of zinc oxide is zinc?
80.34% by mass. Each formula unit contains 1 zinc atom contributing 65.38 g of the 81.379 g total, so 100 g of zinc oxide contains 80.34 g of zinc and a kilogram contains 803.4 g of it.
How many molecules are in one gram of zinc oxide?
About 7.4e+21. One gram is 0.0122882 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 81.379.
How much does a single molecule of zinc oxide weigh?
1.3513e-22 g, which is 81.379 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 oxide it is 81.379. Molar mass carries grams per mole: 81.379 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. Zinc is quoted as 65.38 rather than a whole number for that reason, and the same applies to the other elements here, which is why zinc oxide comes out at 81.379 g/mol rather than 81.
Molar mass, composition and mole equivalents of zinc oxide
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Zinc (Zn) | 1 | 65.38 | 65.38 | 80.34% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 19.66% |
| Total — one mole of zinc oxide | 81.379 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 81.379 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000081 | 0.081379 | 6.022e+17 |
| 10 µmol | 0.00081379 | 0.81379 | 6.022e+18 |
| 100 µmol | 0.0081379 | 8.1379 | 6.022e+19 |
| 1 mmol | 0.081379 | 81.379 | 6.022e+20 |
| 10 mmol | 0.81379 | 813.79 | 6.022e+21 |
| 50 mmol | 4.06895 | 4068.95 | 3.011e+22 |
| 100 mmol | 8.1379 | 8137.9 | 6.022e+22 |
| 250 mmol | 20.3447 | 20,344.7 | 1.506e+23 |
| 500 mmol | 40.6895 | 40,689.5 | 3.011e+23 |
| 750 mmol | 61.0342 | 61,034.2 | 4.517e+23 |
| 1 mol | 81.379 | 81,379 | 6.022e+23 |
| 2 mol | 162.758 | 162,758 | 1.204e+24 |
| 5 mol | 406.895 | 406,895 | 3.011e+24 |
| 10 mol | 813.79 | 813,790 | 6.022e+24 |
Mass is the amount multiplied by 81.379 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 |
|---|---|---|---|
| Silicon dioxide | SiO2 | 60.083 | 16.644 |
| Copper(II) oxide | CuO | 79.545 | 12.572 |
| Titanium dioxide | TiO2 | 79.865 | 12.521 |
| Zinc oxide (this page) | ZnO | 81.379 | 12.288 |
| Manganese dioxide | MnO2 | 86.93604 | 11.503 |
| Aluminium oxide | Al2O3 | 101.9601 | 9.8078 |
| Iron(III) oxide | Fe2O3 | 159.687 | 6.2623 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.