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For the equilibrium A(g) + 2B(g) ⇌ C(g) + D(g), the equilibrium constant is Kc = 10¹². In a 1 L container, the initial amounts of A, B, C, and D are 0.5 mol, 1 mol, 0.5 mol, and 3.5 mol, respectively. What will be the equilibrium concentration of B?

1. 10⁻⁴
2. 2 × 10⁻⁴
3. 4 × 10⁻⁴
4. 8 × 10⁻⁴

Correct answer: 2 × 10⁻⁴

Solution

The equilibrium constant Kc indicates that the reaction heavily favors the products, meaning that as the reaction proceeds towards equilibrium, the concentration of reactants will decrease significantly. Given the initial concentrations and the large value of Kc, the change in concentration of B will be small, leading to an equilibrium concentration of 2 × 10⁻⁴ mol/L.

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