The percentage of copper and oxygen in samples of CuO obtained by different methods were found to be — Mole Concept and Some Basic Concepts of Chemistry Chemistry Question

💡 Solution & Explanation

# Law of Constant Composition **The correct answer is: Law of Constant Composition (or Law of Definite Proportions)** ## Step-by-step reasoning: 1. **Identify what the question states**: Multiple samples of $CuO$ prepared by different methods all show the same percentage composition of Cu and O. 2. **Key observation**: The method of preparation (different synthetic routes) doesn't affect the elemental composition ratios in the final compound. 3. **Apply the law**: The **Law of Constant Composition** states that a pure chemical compound always contains the same elements in the same proportion by mass, regardless of the source or method of preparation. 4. **Mathematical verification**: In $CuO$: - Cu: 64 g/mol, O: 16 g/mol - %Cu = $\frac{64}{80} \times 100 = 80\%$ (always) - %O = $\frac{16}{80} \times 100 = 20\%$ (always) 5. **Why other laws don't apply**: - Law of Multiple Proportions: compares different compounds of same elements - Law of Conservation of Mass: concerns mass before/after reaction - Law of Reciprocal Proportions: relates proportions in three compounds **Conclusion**: The constant percentage composition across different preparation methods is the defining characteristic of the Law of Constant Composition.