In the laboratory preparation of oxygen, the gas cannot be collected by downward displacement of air because

Analysis

The question asks why oxygen gas, when prepared in the laboratory, is typically collected over water rather than by the downward displacement of air. We need to compare the physical properties of oxygen gas and air.

Concept Explanation

In the laboratory, gases are collected based on their physical properties:

1. Upward displacement of air: Used for gases that are less dense than air (e.g., Ammonia, Hydrogen).
2. Downward displacement of air: Used for gases that are denser than air (e.g., Chlorine, Hydrogen Chloride).
3. Over water: Used for gases that are sparingly soluble in water (e.g., Oxygen, Hydrogen).

When we talk about "downward displacement of air," we are essentially saying the gas is heavy enough to push the air out of the top of the collection jar. For this to be effective, there must be a significant difference in density between the gas being collected and the air.

Step-by-Step Solution

1. Density of Oxygen: The molar mass of oxygen ($O_2$) is approximately $16 \times 2 = 32 \text{ g/mol}$.
2. Density of Air: Air is a mixture of gases (mostly Nitrogen and Oxygen). The average molar mass of air is approximately $28.8 \text{ g/mol}$.
3. Comparison: While oxygen is slightly denser than air, the difference is very small. Because their densities are so close, oxygen does not displace air efficiently. Instead of pushing the air out, the oxygen tends to mix with the air due to diffusion.
4. Conclusion: Because the densities are nearly identical, the gas would not be collected in a pure state using the downward displacement of air method.

Identifying the Answer

Looking at the options provided:

• A is incorrect because, while oxygen is slightly denser, the difference is not significant enough to facilitate clean displacement.
• B is correct because the similarity in density causes the gases to mix rather than separate, making the method ineffective.
• C and D are true statements about air, but they do not explain the physical limitation of the collection method.

Exam Tip

When you see questions about gas collection, always remember the "Density Rule":

• If Molar Mass of gas > 29 (approx. density of air), it can be collected by downward displacement of air.
• If Molar Mass of gas < 29, it is collected by upward displacement of air.
• If the Molar Mass is very close to 29 (like Oxygen at 32), the displacement method is inefficient, so we use water collection instead!