Molar MassMolar mass of copper(II) oxide (CuO)
The molar mass of copper(II) oxide, CuO, is 79.55 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 79.55 grams.
79.55 g/mol
63.546 + 15.999 = 79.545 · 2 atoms in the formula · also called cupric oxide
About copper(II) oxide
Copper(II) oxide is a black powder. It is the usual starting material for making copper salts in school, and it is reduced to pink copper by hydrogen.
Other names: cupric oxide.
The working
The formula CuO has 1 copper atom and 1 oxygen atom, 2 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Copper (Cu) | 1 | 63.546 | 63.546 | 79.89% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 20.11% |
63.546 + 15.999 = 79.545 g/mol
With the rounded atomic masses used in many school books (Cu = 63.5, O = 16) the answer is 79.5 g/mol. The difference from 79.55 is too small to matter in most school sums.
Percentage composition
By mass, CuO is 79.89% copper and 20.11% oxygen. The largest share comes from copper. In a 100 g sample there are 79.9 g of copper and 20.1 g of oxygen.
Worked examples
Grams to moles. How many moles are in 5 g of CuO? Divide the mass by the molar mass: 5 ÷ 79.55 = 0.06286 mol.
Moles to grams. What is the mass of 0.25 mol of CuO? Multiply: 0.25 × 79.55 = 19.89 g.
Counting particles. 5 g contains 0.06286 mol × 6.022 × 1023 = 3.785 × 1022 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 7.955 g |
| 0.5 mol | 39.77 g |
| 1 mol | 79.55 g |
| 2 mol | 159.1 g |
| 5 mol | 397.7 g |
| Mass | Moles |
|---|---|
| 1 g | 0.01257 mol |
| 10 g | 0.1257 mol |
| 100 g | 1.257 mol |
| 1 kg | 12.57 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of CuO?
The molar mass of copper(II) oxide (CuO) is 79.55 g/mol, or 79.545 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of CuO?
100 ÷ 79.55 = 1.257 mol.
What is the mass of one formula unit of CuO?
Divide the molar mass by the Avogadro constant: 79.55 ÷ 6.022 × 1023 = 1.321 × 10-22 g.
What is the percentage of copper in CuO?
Copper makes up 79.89% of the mass of CuO: 1 × 63.546 ÷ 79.545 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.