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

# Solution: Law of Definite Proportions **Key Observation:** Different samples of $CuO$ have identical percentages of Cu and O regardless of how they were prepared. **Step 1: Identify what's constant** All samples of copper oxide contain the same mass ratio of copper to oxygen, regardless of the method of synthesis. **Step 2: Recall the relevant laws** - **Law of Definite Proportions (Constant Composition):** A pure chemical compound always contains the same elements in the same mass ratio, regardless of its source or method of preparation. - Law of Multiple Proportions: When two elements form multiple compounds, their mass ratios are in simple whole number ratios. - Law of Conservation of Mass: Mass is neither created nor destroyed. **Step 3: Match the observation to the law** Since $CuO$ samples from different preparation methods show identical Cu and O percentages, this directly demonstrates that a pure compound ($CuO$) has a fixed composition. $$\text{Cu : O mass ratio in all CuO samples} = \text{constant}$$ **Answer: Law of Definite Proportions (or Law of Constant Composition)** This law is foundational to stoichiometry and proves that compounds are pure substances with invariant composition, not mixtures.