Going from grams to moles for zinc oxide means dividing by its molar mass, 81.379 g/mol. One gram is 0.0122882 mol, or 12.2882 mmol; 100 mg is 1.2288 mmol; and a 81.379 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, ZnO, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.1229 mmol of zinc oxide; on a three-decimal balance it is 1 mg, or 12.29 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.
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 you weighed, in grams
81.379- The molar mass of zinc oxide 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 81.379 g/mol, the molar mass of zinc oxide.
- 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 zinc oxide in moles
0.25 ÷ 81.379 = 0.00307205 mol, or 3.072 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 12.29 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of zinc oxide in moles
2 ÷ 81.379 = 0.0245764 mol, or 24.576 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 12.29 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of zinc oxide in moles
25 ÷ 81.379 = 0.307205 mol, or 307.2 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 12.29 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 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 do I convert grams of zinc oxide to moles?
Divide the mass in grams by 81.379 g/mol. So 1 g is 0.0122882 mol, 10 g is 0.122882 mol and 100 g is 1.22882 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of zinc oxide?
1.22882 mol, which is 1228.82 mmol. Read the other way, one mole of zinc oxide is 81.379 g, so 100 g is 1.229 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of zinc oxide is 12.288 µmol, 10 mg is 122.88 µmol and 100 mg is 1.2288 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 zinc oxide, which is 0.0120424 mol rather than 0.0122882 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 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000012 | 0.0122882 | 12.2882 |
| 5 mg | 0.000061 | 0.0614409 | 61.4409 |
| 10 mg | 0.000122882 | 0.122882 | 122.882 |
| 25 mg | 0.000307205 | 0.307205 | 307.205 |
| 50 mg | 0.000614409 | 0.614409 | 614.409 |
| 100 mg | 0.00122882 | 1.22882 | 1228.82 |
| 250 mg | 0.00307205 | 3.07205 | 3072.05 |
| 500 mg | 0.00614409 | 6.14409 | 6144.09 |
| 1 g | 0.0122882 | 12.2882 | 12,288.2 |
| 2 g | 0.0245764 | 24.5764 | 24,576.4 |
| 5 g | 0.0614409 | 61.4409 | 61,440.9 |
| 10 g | 0.122882 | 122.882 | 122,882 |
| 25 g | 0.307205 | 307.205 | 307,205 |
| 50 g | 0.614409 | 614.409 | 614,409 |
| 100 g | 1.22882 | 1228.82 | 1228818.245493 |
| 250 g | 3.07205 | 3072.05 | 3072045.613733 |
| 500 g | 6.14409 | 6144.09 | 6144091.227467 |
Every row is the mass divided by 81.379 g/mol. A balance reading to 1 mg resolves 12.29 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 |
|---|---|---|---|
| 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.