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At a certain temperature Kc = 1.0 for the reaction NO2(g) + NO(g) ⇌ N2O(g) + O2(g). Equal numbers of moles of NO and NO2 are placed in a 5 litre vessel and allowed to reach equilibrium, at which point the concentration of N2O is 0.5 M. How many total moles of (NO + NO2) were initially introduced into the container?

1. 10
2. 7.5
3. 5
4. 2.5

Correct answer: 10

Solution

With Kc = 1 and [N2O] = [O2] = 0.5 M, the equilibrium [NO] = [NO2] = 0.5 M, so each initial concentration was 1 M, i.e. 5 moles each, totalling 10 moles.

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