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 **Step 1: Identify what the question states** Different samples of $CuO$ (obtained by different methods) have the same percentage composition of Cu and O. **Step 2: Recall the relevant laws of chemistry** - **Law of Constant Composition (Definite Proportions)**: A pure chemical compound always contains the same elements in the same percentage by mass, regardless of how it is prepared. - Law of Conservation of Mass: Matter cannot be created or destroyed - Law of Multiple Proportions: When two elements form multiple compounds, the mass ratios are simple whole numbers **Step 3: Connect to the given observation** The key phrase is "**samples... obtained by different methods**" yet show "**the same percentage composition**." This directly demonstrates that the composition is fixed and independent of the preparation method. **Step 4: Conclusion** This observation perfectly illustrates the **Law of Constant Composition** (or **Definite Proportions**), which states that all pure samples of a compound contain the same elements in identical mass percentages. For example, every pure $CuO$ sample will always contain: - Cu: ~79.9% - O: ~20.1% This constancy is what makes a compound "pure" and chemically definite.