2019 O Level Chem P1 Q1 — Gas Preparation / Drying and Collecting Gases Solution

Answer: A (calcium oxide drying, downward displacement of air). The gas turns moist red litmus paper blue, is soluble in water and is less dense than air, so it is ammonia, NH₃. Two things must be right: the drying agent and the collection method.

Drying: ammonia is a basic (alkaline) gas, so it must not be passed through an acidic drying agent. Concentrated sulfuric acid would react with and absorb the ammonia, which rules out the options using it. Calcium oxide, CaO, is a basic solid and is the correct drying agent for ammonia.

Collection: because ammonia is very soluble in water it cannot be collected over water, and because it is less dense than air it is collected by downward displacement of air, in an inverted gas jar with the mouth pointing down. Only A combines CaO drying with an inverted gas jar.