Question
Measurements show that an unknown compound X has the following composition by mass:
Composition by mass
| Carbon | 42.9% |
|---|---|
| Oxygen | 57.2% |
Write the empirical chemical formula of X. Show the conversion from mass percentages to moles and reduce the mole ratio to the smallest whole numbers. Use appropriate significant figures.
Answer
Use a 100 g sample so that each percentage becomes the same numerical mass in grams.
| Element | Mass in 100 g | Molar mass | Amount |
|---|---|---|---|
| C | 42.9 g | 12.011 g/mol | 3.57 mol |
| O | 57.2 g | 15.999 g/mol | 3.58 mol |
Now divide both amounts by the smaller value, 3.57 mol:
- C:
- O:
The smallest whole-number ratio is therefore .
Empirical formula: .
Evidence boundary
The calculation assumes the sample contains only carbon and oxygen and that the reported percentages are rounded measurements. Their total is 100.1%, a 0.1-percentage-point rounding difference that does not change the 1:1 mole ratio. The result is an empirical formula only; a molecular formula would require molar-mass data.
Sources
These references support the concepts and methods used in the explanation above.