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For the equilibrium 2SO2(g) + O2(g) <-> 2SO3(g) in a 10 L flask, Kc = 100 mol⁻¹ L. (a) If the moles of SO3 equal the moles of SO2, how many moles of O2 are present? (b) If the moles of SO3 are twice the moles of SO2, how many moles of O2 are present?

1. (a) 1 mol, (b) 2 mol
2. (a) 2 mol, (b) 1 mol
3. (a) 1/2 mol, (b) 1 mol
4. (a) 1 mol, (b) 1/2 mol

Correct answer: (a) 1 mol, (b) 2 mol

Solution

From Kc = [SO3]²/([SO2]²[O2]) the oxygen concentration is (ratio)²/Kc; doubling the SO3:SO2 ratio quadruples the required O2, so part (b) needs more O2 than part (a).

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