Why is concentrated sulfuric acid not used to dry ammonia gas?

• A. Because it will react with ammonia, which is a neutralisation reaction.
• B. Because it cannot absorb water from ammonia.
• C. Because it would produce toxic chlorine gas.
• D. Because it is too weak to remove water from ammonia.

Answer: (A)

Concentrated sulfuric acid is a strong drying agent because it removes water. But with ammonia gas, the situation is different: ammonia is a base, and the acid-base reaction between a strong acid and a base occurs readily. When ammonia meets concentrated sulfuric acid, they neutralize each other to form a salt such as ammonium sulfate (for example, ammonium sulfate salts like ammonium hydrogen sulfate can form depending on stoichiometry). In this case, the acid is consumed in the neutralisation reaction rather than acting as a drying agent to take up water molecules. So it wouldn’t dry the ammonia effectively; instead, you’d end up with a salt and residual water, and the gas would no longer be dried cleanly. That’s why concentrated sulfuric acid isn’t used to dry ammonia gas.