Moles to grams for zinc oxide is a multiplication by 81.379 g/mol. A mole weighs 81.379 g, 100 mmol weighs 8.1379 g, 10 mmol weighs 0.81379 g and a millimole is 81.379 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 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.
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.
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.
Purity is the usual gap between the calculation and the balance. A reagent sold at 98% means 1.6276 g of a nominal 81.379 g is something else, so for exact work you divide the weighed mass by the assay figure on the certificate. For most purposes the difference is smaller than the error in reading the meniscus, but it is systematic rather than random, so it does not average out.
The formula
n- The amount of zinc oxide you need, in moles
81.379- The molar mass of zinc oxide 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 81.379 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 oxide in grams
5 mmol × 81.379 g/mol = 0.406895 g, which is 406.895 mg. Doubling the amount doubles the mass to 0.81379 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of zinc oxide in grams
250 mmol × 81.379 g/mol = 20.3447 g, which is 20,344.7 mg. Doubling the amount doubles the mass to 40.6895 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of zinc oxide in grams
2 mol × 81.379 g/mol = 162.758 g, which is 162,758 mg. Doubling the amount doubles the mass to 325.516 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 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 grams is one mole of zinc oxide?
81.379 g, by definition of the molar mass. Two moles is 162.758 g and half a mole is 40.6895 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of zinc oxide is 8.1379 g. One millimole is 81.379 mg and one micromole is 81.379 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 0.8138 mg of zinc oxide, weigh 8.138 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 anhydrous form, ZnO, at 81.379 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. Salts that are sold as hydrates weigh more per mole because the water is part of the crystal.
Moles to grams 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000081 | 0.081379 | 0.000083 |
| 10 µmol | 0.00081379 | 0.81379 | 0.000830066 |
| 100 µmol | 0.0081379 | 8.1379 | 0.00830066 |
| 1 mmol | 0.081379 | 81.379 | 0.0830066 |
| 10 mmol | 0.81379 | 813.79 | 0.830066 |
| 50 mmol | 4.06895 | 4068.95 | 4.15033 |
| 100 mmol | 8.1379 | 8137.9 | 8.30066 |
| 250 mmol | 20.3447 | 20,344.7 | 20.7516 |
| 500 mmol | 40.6895 | 40,689.5 | 41.5033 |
| 750 mmol | 61.0342 | 61,034.2 | 62.2549 |
| 1 mol | 81.379 | 81,379 | 83.0066 |
| 2 mol | 162.758 | 162,758 | 166.013 |
| 5 mol | 406.895 | 406,895 | 415.033 |
| 10 mol | 813.79 | 813,790 | 830.066 |
Weigh is the amount multiplied by 81.379 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 |
|---|---|---|---|
| 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.