Ka, pKa and weak acid pHEdexcel A-Level Chemistry: Subtopic test

1. 1
   A food technologist analyses a 0.100 mol dm⁻³ solution of ethanoic acid, CH₃COOH, a weak monobasic acid, at 298 K. For ethanoic acid, $K_a$ = 1.74 × 10⁻⁵ mol dm⁻³.

   (a)

   Which expression gives the acid dissociation constant, $K_a$, for ethanoic acid in aqueous solution?

   [1 mark]

   • A$\frac{[CH_3COOH]}{[H^+][CH_3COO^-]}$
   • B$\frac{[H^+][CH_3COO^-]}{[H_2O][CH_3COOH]}$
   • C$\frac{[H^+][CH_3COO^-]}{[CH_3COOH]}$
   • D$[H^+][CH_3COO^-]$

   (b)

   What is the pKₐ of ethanoic acid?

   [1 mark]

   • A4.76
   • B9.24
   • C1.74
   • D−4.76

   (c)

   Calculate the pH of the 0.100 mol dm⁻³ ethanoic acid solution. State the assumptions you make.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist prepares a 0.0500 mol dm⁻³ solution of methanoic acid, HCOOH, for use as a descaling agent. For methanoic acid, $K_a$ = 1.78 × 10⁻⁴ mol dm⁻³ at 298 K. She compares it with propanoic acid, which has pKₐ = 4.87.

   (a)

   The chemist calculates the pH of the methanoic acid using $[H^+] = \sqrt{K_a c}$. Which pair of assumptions is needed for this expression?

   [1 mark]

   • AThe acid is fully dissociated, and $[H^+]$ equals the initial acid concentration
   • B$[H^+] = [HCOO^-]$ (water's ionisation is negligible), and the equilibrium $[HCOOH]$ is approximately its initial concentration
   • C$K_a$ is zero, and the solution is at 25 °C
   • D$[H^+]$ is much greater than $[HCOO^-]$, and the equilibrium $[HCOOH]$ is zero

   (b)

   Which statement about the two acids is correct?

   [1 mark]

   • APropanoic acid is the stronger acid because its pKₐ is larger
   • BThe two acids have the same $K_a$ because both are weak
   • CPropanoic acid has the larger $K_a$ because its pKₐ is larger
   • DMethanoic acid is the stronger acid because its pKₐ (3.75) is lower, corresponding to a larger $K_a$

   (c)

   The chemist states that the pH of the methanoic acid solution is greater than that of a 0.0500 mol dm⁻³ solution of a strong monobasic acid. Explain why.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student dissolves 0.440 g of a pure solid weak monobasic acid, HA (molar mass 88.0 g mol⁻¹), in water and makes the solution up to 100 cm³. A calibrated pH meter reads 3.06 at 298 K.

   (a)

   Calculate $K_a$ for the acid HA. State the assumptions you make.

   [3 marks]

   (b)

   A second student finds $K_a$ for the same acid by titration with sodium hydroxide and a pH meter. Describe how $K_a$ can be found from the results, and explain why the method works.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist compares two 0.100 mol dm⁻³ acids at 298 K: ethanoic acid (CH₃COOH, $K_a$ = 1.74 × 10⁻⁵ mol dm⁻³) and chloroethanoic acid (CH₂ClCOOH, $K_a$ = 1.38 × 10⁻³ mol dm⁻³). Both are weak monobasic acids.

   (a)

   A 25.0 cm³ sample of 0.250 mol dm⁻³ ethanoic acid is diluted to 250 cm³. Calculate the pH of the solution before and after dilution, and explain why the pH does not rise by exactly 1.

   [6 marks]

   (b)

   Evaluate whether $K_a$ and pKₐ allow the strengths of the two acids to be compared, and whether the approximation $[HA]_{eq} \approx [HA]_{initial}$ is equally valid for both when calculating pH.

   [6 marks]

   Total for question 4: 12 marks

End of questions