AQA A-Level Chemistry Paper 3, 2023: Question 13

When HF is added to water at 298 K, this equilibrium is established:

HF(aq) ⇌ H⁺(aq) + F⁻(aq)

At equilibrium, [HF] = 7.70 × 10⁻³ mol dm⁻³ and [F⁻] = [H⁺] = 2.30 × 10⁻³ mol dm⁻³.

What is the value of the equilibrium constant (K_c), in mol dm⁻³, at 298 K?

• A: 1.45 × 10³
• B: 3.35
• C: 2.99 × 10⁻¹
• D: 6.87 × 10⁻⁴

The correct equilibrium constant is:

K_c = 6.87 × 10⁻⁴ mol dm⁻³

Step 1: Write the Expression for K_c

K_c = [H⁺][F⁻] / [HF]

Step 2: Substitute the Values

• [H⁺] = 2.30 × 10⁻³ mol dm⁻³
• [F⁻] = 2.30 × 10⁻³ mol dm⁻³
• [HF] = 7.70 × 10⁻³ mol dm⁻³

K_c = (2.30 × 10⁻³) × (2.30 × 10⁻³) ÷ (7.70 × 10⁻³)

Step 3: Perform the Calculation

1. Multiply the numerator: (2.30 × 10⁻³) × (2.30 × 10⁻³) = 5.29 × 10⁻⁶
2. Divide by the denominator: 5.29 × 10⁻⁶ ÷ 7.70 × 10⁻³ = 6.87 × 10⁻⁴

K_c = 6.87 × 10⁻⁴ mol dm⁻³

• A: This value is far too large and likely results from squaring or multiplying incorrectly.
• B: This could come from not squaring the numerator or using incorrect powers of 10.
• C: This is closer to the correct answer but could result from an error in dividing by the denominator.

The equilibrium constant is:

D: 6.87 × 10⁻⁴ mol dm⁻³