A Levels Chemistry (9701)•9701/13/M/J/19
Explanation
Gases forming acids with water lower pH Steps:
- Lowering pH requires the gas to produce H+ ions via acid formation in water.
- Classify each gas by its reaction type: acidic, basic, or neutral.
- Acidic gases like non-metal oxides react to form acids; basic gases form bases.
- Select the gas that definitively forms an acidic solution.
Why C is correct:
- Nitrogen dioxide (NO2) reacts with water to form nitric acid (HNO3, strong acid) and nitrous acid (HNO2): 3NO2 + H2O → 2HNO3 + NO, releasing H+ and lowering pH.
Why the others are wrong:
- A. Ammonia (NH3) forms NH4OH, a weak base, raising pH by producing OH-.
- B. Sulfur dioxide (SO2) forms weak H2SO3, but equilibrium limits H+ production, not significantly lowering pH here.
- D. Not enough information (option incomplete).
Final answer: C
Topic: Nitrogen and sulfur
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