Meaning in general chemistry
“How many grams of oxygen in Cu2O” commonly refers to the oxygen portion of copper(I) oxide by formula mass (grams of O per mole of Cu2O) and, equivalently, the mass percent of oxygen in Cu2O.
Formula composition of Cu2O
One formula unit of Cu2O contains:
- Copper atoms: 2
- Oxygen atoms: 1
Molar mass of Cu2O
Using standard average atomic masses (Cu = 63.546 g/mol, O = 15.999 g/mol), the molar mass is:
| Element | Count | Atomic mass (g/mol) | Contribution (g/mol) |
|---|---|---|---|
| Cu | 2 | 63.546 | \(2 \cdot 63.546 = 127.092\) |
| O | 1 | 15.999 | \(1 \cdot 15.999 = 15.999\) |
| Total | — | — | \(127.092 + 15.999 = 143.091\) |
Grams of oxygen in one mole of Cu2O
The oxygen contribution in the table is the oxygen mass per mole of Cu2O: 15.999 g of oxygen per 1 mol of Cu2O.
1 mol Cu2O contains 15.999 g O and 127.092 g Cu, totaling 143.091 g.
Oxygen mass percent in Cu2O
Mass percent follows directly from the fraction of the molar mass due to oxygen:
The complementary copper fraction is \(\approx 88.82\%\), consistent with copper’s larger atomic mass and the subscript 2.
Oxygen grams in a given mass of Cu2O
A mass-based interpretation uses the oxygen mass fraction:
Example value (typical lab rounding):
- Sample mass \(m(\mathrm{Cu_2O}) = 25.0\ \mathrm{g}\): \(m_{\mathrm{O}} \approx 25.0 \cdot 0.1118 = 2.80\ \mathrm{g}\)
Mass-composition visualization
Frequent errors and consistency checks
- Subscript mismatch: Cu2O has one oxygen atom per formula unit, not two.
- Unit mismatch: “grams of oxygen” refers to oxygen’s mass contribution, not gaseous O2.
- Reasonableness check: oxygen’s percent is well below 50% because copper is heavy and appears twice in the formula.