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: Analyze the observation** Different samples of $CuO$ (copper oxide) obtained by different methods all contain the same percentage of copper and oxygen by mass. **Step 2: Identify what this means** Regardless of *how* the compound was prepared (different methods, different sources), the mass ratio of elements remains constant. **Step 3: Match to fundamental law** This is the defining characteristic of the **Law of Constant Composition** (also called Law of Definite Proportions): > *A pure chemical compound always contains the same elements in the same proportion by mass, regardless of its source or method of preparation.* **Step 4: Why this law applies** - $CuO$ always has a fixed 1:1 molar ratio of Cu to O atoms - This fixed atomic ratio produces a fixed mass ratio: $\frac{m(Cu)}{m(O)} = \frac{64}{16} = 4:1$ (constant) **Answer: A is correct because the constant percentages of elements in different samples of the same pure compound directly illustrate the Law of Constant Composition.**