zno (ZnO): name, ions, oxidation states, molar mass, and percent composition
The formula ZnO indicates a compound built from zinc and oxygen in a 1:1 ratio. In general chemistry, this is treated as a binary ionic compound unless additional context suggests a molecular species.
Goal. Determine (1) the correct name of zno, (2) the ions and oxidation states, and (3) the molar mass and mass percent composition.
1) Classify the compound and name it
- Identify element types: Zn is a metal; O is a nonmetal. Metal + nonmetal commonly forms an ionic compound.
- Apply binary ionic naming: use the metal name followed by the nonmetal root + “-ide”. Oxygen becomes “oxide”.
Name: ZnO is zinc oxide.
2) Determine ions and oxidation states
Oxygen in binary ionic compounds is almost always oxide with charge \( -2 \). To make the overall formula neutral, zinc must be \( +2 \).
3) Calculate molar mass (formula mass) of ZnO
Use standard atomic masses (to typical textbook precision): Zn \(= 65.38\ \text{g·mol}^{-1}\), O \(= 16.00\ \text{g·mol}^{-1}\).
| Element | Atoms in ZnO | Atomic mass (g·mol\(^{-1}\)) | Mass contribution (g·mol\(^{-1}\)) |
|---|---|---|---|
| Zn | 1 | 65.38 | 65.38 |
| O | 1 | 16.00 | 16.00 |
| Total | — | — | 81.38 |
\[ M(\text{ZnO}) = 1(65.38) + 1(16.00) = 81.38\ \text{g·mol}^{-1} \]
4) Calculate percent composition of Zn and O in ZnO
Mass percent is “part over whole” multiplied by \(100\%\).
\[ w_{\mathrm{Zn}} = \frac{65.38}{81.38}\times 100\% = 80.34\% \]
\[ w_{\mathrm{O}} = \frac{16.00}{81.38}\times 100\% = 19.66\% \]
Interpretation (law of constant composition). Any pure sample of zinc oxide (ZnO) has the same fixed mass ratio \(65.38:16.00\) and therefore the same percent composition (within measurement and rounding).
5) Helpful visualization: ionic ratio in ZnO
Final results for ZnO
- Name: zinc oxide
- Ions / oxidation states: Zn\(^{2+}\) and O\(^{2-}\); Zn \(+2\), O \( -2 \)
- Molar mass: \(81.38\ \text{g·mol}^{-1}\)
- Percent composition: Zn \(80.34\%\), O \(19.66\%\)
\[ 2\,\mathrm{Zn}(s) + \mathrm{O}_2(g) \rightarrow 2\,\mathrm{ZnO}(s) \]