The correct answer is B because:

The equation required to calculate the pH of a buffer is:
- $[H^{+}] = K_{a} \frac{[acid]}{[salt]}$
The concentrations of the acid and salt are required
Moles of 2.00 mol dm^-3 acid in 200 cm³ is 0.4 moles
- So [acid] = $\frac{0.4}{800} \times 1000$ = 0.5 mol dm^-3

Moles of salt in 600 cm³is 0.6
- So [salt] = $\frac{0.6}{800} \times 1000$ = 0.75 mol dm^-3
$[H^{+}] = K_{a} \frac{[acid]}{[salt]} = 1.32 x 10^{- 5} x \frac{0.5}{0.75} = 8.8 x 10^{- 6} mol {dm}^{- 3}$
pH = 5.055 / 5.06
The volume will cancel out in this case, however it is correct and good practice to calculate the concentrations of the acid and salt first

A is incorrect as the concentrations from the question have been used to calculate the pH of the buffer, the number of moles and subsequent concentrations have not been used

$[H^{+}] = K_{a} \frac{[acid]}{[salt]} = 1.32 x 10^{- 5} x \frac{2}{1} = 2.64 x 10^{- 5} mol {dm}^{- 3}$

taking the -log₁₀ of this gives a pH value of 4.57

B is incorrect as the wrong equation has been used to calculate [H⁺]

$[H^{+}] = K_{a} \frac{[salt]}{[acid]} = 1.32 x 10^{- 5} x \frac{0.75}{0.5} = 1.98 x 10^{- 5} mol {dm}^{- 3}$

Taking the -log₁₀ of this gives a pH value of 4.70

D is incorrect as the -log₁₀ of the K_a has been used to calculate pH

-log₁₀1.32 x 10^-5 = 4.88

Indicator	pH range for colour change
Methyl orange	3.2 - 4.4
Phenolphthalein	8.2 - 10.0
Thymol blue	1.2 - 2.8
Bromothymol blue	6.0 - 7.6

Acids, Bases & Buffers (OCR A Level Chemistry)

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