If humid air is polluted by chlorine discharged, the air can be restored by sprinkling

Analyze the question: Chlorine gas has polluted humid air. We need a substance that, when sprinkled, removes the chlorine by reacting with it. Chlorine is an oxidizing agent, so we need a reducing agent to convert Cl₂ into harmless chloride ions (Cl⁻).

Concept: Cl₂ + 2e⁻ → 2Cl⁻. Any substance that readily gives up electrons will remove it.

Checking each option:

• A. Solid MnO₂: This is an oxidizing agent. It is used to produce chlorine from concentrated HCl, so it cannot remove it.
• B. Acidified KMnO₄: This is also a strong oxidizing agent. It reacts with HCl to liberate chlorine, so it would make things worse.
• C. Acidified FeSO₄: Fe²⁺ is a reducing agent. It reduces chlorine to chloride: 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻ This removes the chlorine from the air.
• D. Saturated NaCl(aq): Chlorine is much less soluble in brine because of the common-ion effect, which is why chlorine is collected over brine. It does not chemically remove chlorine, so it is not a way to restore the air.

Tip: Gas is an oxidizer, so you want a reducing agent. Remember that Fe²⁺ (green) → Fe³⁺ (yellow-brown) is a classic test of oxidizing agents. When you see "remove an oxidizing gas," look for the reducing agent among the options. Both MnO₂ and KMnO₄ are oxidizers, so you can eliminate them quickly.