Which of the following gases may not be dried with concentrated sulphuric acid?

Analysis

The question asks us to identify which of the listed gases cannot be dried using concentrated tetraoxosulphate(VI) acid ($H_2SO_4$). To answer this, we must understand the chemical property of concentrated $H_2SO_4$ as a drying agent.

Concept

Concentrated $H_2SO_4$ is a powerful acidic drying agent. In chemistry, when choosing a drying agent, the rule is: the drying agent must not react chemically with the gas being dried.

• If a gas is acidic, it can be dried using acidic drying agents like concentrated $H_2SO_4$ or phosphorus(V) oxide ($P_2O_5$).
• If a gas is basic (alkaline), it will react with an acidic drying agent (neutralization reaction) and therefore cannot be dried using it.

Step-by-Step Solution

1. Analyze the nature of the gases: • HCl (Hydrogen chloride): This is an acidic gas. It does not react with $H_2SO_4$. • NH$_3$ (Ammonia): This is a basic (alkaline) gas. • Cl$_2$ (Chlorine): This is a neutral/acidic gas. It does not react with $H_2SO_4$. • SO$_2$ (Sulphur dioxide): This is an acidic gas. It does not react with $H_2SO_4$.

2. Evaluate the reaction with $H_2SO_4$: • Since Ammonia ($NH_3$) is a base, it reacts readily with concentrated $H_2SO_4$ (an acid) to form a salt (Ammonium tetraoxosulphate(VI)): $$2NH_{3(g)} + H_2SO_{4(conc)} \rightarrow (NH_4)_2SO_{4(s)}$$ • Because the gas is consumed in this reaction, it cannot be dried using this method.

Conclusion

Because ammonia is basic, it undergoes a neutralization reaction with concentrated sulphuric acid. Therefore, it is the gas that cannot be dried using this substance.

Correct Option: B

Exam Tip

Remember the "Acid-Base" rule for drying gases:

• Acidic gases (e.g., $CO_2$, $SO_2$, $HCl$) are dried with acidic agents (conc. $H_2SO_4$).
• Basic gases (e.g., $NH_3$) are dried with basic agents (e.g., Calcium oxide, $CaO$).
• Neutral gases (e.g., $H_2$, $O_2$, $N_2$) can generally be dried with either.