In the laboratory preparation of chlorine from concentrated hydrochloric acid in the presence of potassuim…

Analysis of the Question

The question asks for the drying agent used to remove moisture from chlorine gas ($Cl_2$) produced in the laboratory. Chlorine is an acidic gas, so the drying agent must be one that does not react with it.

Concept Explanation

In the laboratory, chlorine is prepared by the reaction of concentrated hydrochloric acid ($HCl$) with potassium tetraoxomanganate(VII) ($KMnO_4$). The gas produced is usually moist. To dry a gas, we pass it through a substance that absorbs water but does not chemically react with the gas being dried.

• Concentrated tetraoxosulphate(VI) acid ($H_2SO_4$): This is a powerful dehydrating agent. It is acidic and does not react with chlorine gas, making it the standard choice for drying chlorine.
• Calcium oxide ($CaO$): This is a basic oxide. It would react with chlorine gas to form calcium chloride and calcium hypochlorite.
• Sodium hydroxide ($NaOH$): This is an alkali. It reacts readily with chlorine gas to form sodium chloride and sodium oxochlorate(I) (bleach).
• Anhydrous calcium chloride ($CaCl_2$): While often used as a drying agent, it is generally used for neutral or acidic gases, but in the specific context of the laboratory preparation of chlorine, concentrated $H_2SO_4$ is the preferred industrial and laboratory standard for drying.

Step-by-Step Solution

1. Identify the gas: The gas is chlorine ($Cl_2$), which is acidic.
2. Evaluate the options: • D: Sodium hydroxide is a base; it will absorb the chlorine gas (neutralization), so it cannot be used. • C: Calcium oxide is a base; it will react with the chlorine gas. • B: Anhydrous calcium chloride is a drying agent, but concentrated tetraoxosulphate(VI) acid is the specific reagent used in the standard laboratory setup for this preparation to ensure the gas is effectively dried without interference. • A: Concentrated tetraoxosulphate(VI) acid ($H_2SO_4$) is acidic and has a high affinity for water. It effectively removes moisture from the chlorine gas without reacting with it.

Conclusion

Concentrated tetraoxosulphate(VI) acid is the standard drying agent for chlorine gas in this preparation.

Exam Tip

Always remember the "Acid-Base" rule for drying gases:

• Acidic gases (like $Cl_2$, $SO_2$, $HCl$) are dried using acidic drying agents (like conc. $H_2SO_4$).
• Basic gases (like $NH_3$) are dried using basic drying agents (like calcium oxide).
• Neutral gases (like $O_2$, $N_2$) can be dried using a variety of agents.