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A gaseous equilibrium A(g) ⇌ 2B(g) initially contains 2.0 mol of A and 4.0 mol of B at equilibrium in a container of volume V at temperature T. An additional 2.0 mol of A is introduced into the same container, and the system re-attains equilibrium at the same volume V and temperature T. After this new equilibrium is reached, the volume of the container is suddenly doubled (to 2V) at constant temperature, and the system re-attains equilibrium again. How many moles of B are present at this final equilibrium?

1. 3.86
2. 5.86
3. 4.86
4. 6.58

Correct answer: 6.58

Solution

Setting V=1 L, Kc=8. After adding 2 mol A: solving (4+2x)²/(4-x) = 8 gives x ≈ 0.606, so n(B) ≈ 5.21 mol. Doubling the volume shifts equilibrium forward; solving again yields n(B) ≈ 6.58 mol.

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