9701 MCQ Solution 11

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Question 11: [Physical > Equilibria]:

In an experiment, b mol of hydrogen iodide were put into a sealed vessel under pressure p. At equilibrium, x mol of the hydrogen iodide had dissociated, the reaction being represented by the following equation.

2HI(g) ó H₂(g) + I₂(g)

Which expression for K_p is correct?

A (x²)/(b – x)²

B (x²p²)/(b – x)²

C (x²p²)/(4b)(b – x)

D (x²)/4(b – x)²

Reference: Past Exam Paper – November 2002 Paper 1 Q11

Solution 11:

Answer: D.

The equation for calculating K_p:

K_p = p(products)/p(reactants)

Equilibrium mols of H₂ = 0.5x because of the chemical equation

Equilibrium mols of I₂ = 0.5x because of the chemical equation

Equilibrium mols of HI = b – x because this will be the mols remaining after reaching equilbrium

p(products) = (0.5x)(0.5x) = 0.25x²

p(reactants) = (b – x)²

Therefore K_p = (0.25x²)/ (b – x)² = (x²)/4(b – x)²