In the laboratory preparation of oxygen, dried oxygen is usually collected over

Analysis of the Question

The question asks for the appropriate method of collecting dried oxygen gas in a laboratory setting. We need to identify which substance or condition allows for the collection of oxygen without reacting with it or being displaced by it in a way that compromises the gas.

Concept Explanation

In the laboratory, oxygen is typically prepared by the thermal decomposition of potassium trioxochlorate(V) ($KClO_3$) using manganese(IV) oxide ($MnO_2$) as a catalyst.

When collecting gases, the method depends on the gas's solubility and density:

1. Downward displacement of water: Used for gases that are only slightly soluble in water (like oxygen). However, if the gas must be dried, collecting it over water introduces moisture.
2. Upward displacement of air: Used for gases that are less dense than air.
3. Collection over mercury: Mercury is a dense liquid metal that does not react with oxygen. Because oxygen is not soluble in mercury and does not react with it, mercury is an ideal medium for collecting gases that might otherwise react with water or other common laboratory liquids.

Step-by-Step Solution

1. Evaluate Option A (Hydrochloric acid): Oxygen is a gas, and hydrochloric acid is a liquid. Oxygen does not react with it, but it is not a standard medium for gas collection, and the gas would simply bubble through and escape into the atmosphere.
2. Evaluate Option B (Mercury): Oxygen is chemically inert toward mercury at room temperature. Because mercury is very dense and does not dissolve oxygen, it provides a perfect seal for collecting the gas without introducing water vapor. This is the standard laboratory method for collecting dry oxygen.
3. Evaluate Option C (Calcium chloride): Calcium chloride ($CaCl_2$) is a drying agent (desiccant), not a medium for collecting gas. You would pass the oxygen through a U-tube containing calcium chloride to dry it, but you cannot collect a gas "over" a solid.
4. Evaluate Option D (Tetraoxosulphate(VI) acid): Concentrated tetraoxosulphate(VI) acid ($H_2SO_4$) is also a drying agent. Like calcium chloride, it is used to remove moisture from the gas, not as a collection medium.

Conclusion

Since oxygen is collected over a liquid that does not react with it and does not introduce moisture, mercury is the correct choice.

Exam Tip: Always distinguish between drying agents (like $CaCl_2$ and conc. $H_2SO_4$) and collection media (like water or mercury). If a question asks how to dry a gas, look for a desiccant; if it asks how to collect a gas, look for a liquid displacement method.