Question detail
In a sports injury pack, the endothermic reaction absorbs 25 kJ of energy. If the pack contains 2.5 mol of ammonium nitrate, how many grams of ammonium nitrate are required? (Mr = 80.05 g mol⁻¹).
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Exothermic and endothermic reactions
Question
- A. 20.0 g
- B. 40.0 g
- C. 50.0 g
- D. 60.0 g
Answer
40.0 g
Explanation
Energy absorbed per mole = 25 kJ/2.5 mol = 10 kJ mol⁻¹. Mass = 2.5 mol × 80.05 g mol⁻¹ = 200.1 g, but only 25 kJ required, so 2.5 mol is too much. Correct calculation gives 40.0 g.
Common mistake
Misidentifying the temperature change in sports injury packs
Students think sports injury packs warm the surroundings because they feel hot
Explain that sports injury packs are endothermic; they absorb heat from the surroundings, so the pack feels cold and the surrounding temperature falls
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