Assertion: Effusion rate of oxygen is smaller than nitrogen.
Reason: Molecular size of nitrogen is smaller than oxygen.

Options

A. If both assertion and reason are true and reason is the explaination of assertion.
B. If both assertion and reason are true but reason is not the correct explaination of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that "the effusion rate of oxygen is smaller than that of nitrogen."
- **Understanding Effusion**: Effusion is the process by which gas molecules escape through a tiny hole into a vacuum. According to Graham's law of effusion, the rate of effusion of a gas is inversely proportional to the square root of its molar mass (molecular weight).
- **Molecular Weights**: The molar mass of nitrogen (N₂) is approximately 28 g/mol, while the molar mass of oxygen (O₂) is approximately 32 g/mol. Since oxygen has a higher molar mass than nitrogen, it will effuse more slowly.

**Conclusion**: The assertion is true because the effusion rate of oxygen is indeed smaller than that of nitrogen.

### Step 2: Analyze the Reason
The reason states that "the molecular size of nitrogen is smaller than oxygen."
- **Understanding Molecular Size**: The term "molecular size" can refer to several factors, including molecular weight and atomic radii. In this context, it is more relevant to consider molecular weight rather than size.
- **Comparison of Molecular Sizes**: The molecular weight of nitrogen is less than that of oxygen, but the statement incorrectly implies that nitrogen has a smaller molecular size in terms of atomic radii. In fact, the atomic radius of nitrogen is larger than that of oxygen.

**Conclusion**: The reason is false because it inaccurately states the relationship between the molecular sizes of nitrogen and oxygen.

### Final Conclusion
- The assertion is true: the effusion rate of oxygen is smaller than that of nitrogen.
- The reason is false: the molecular size of nitrogen is not smaller than that of oxygen.

### Correct Answer
Thus, the correct option is that the assertion is true, but the reason is false.

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