Calculate pH of Buffer Solution & Understand Ka Equilibrium

[FeDeX_LaTeX]

Homework Statement

A student sets out to make a buffer solution. The student measures out 27 cm³ of 0.050 mol dm^-3 HA solution and reacts with one-third of the volume of 0.10 mol dm^-3 sodium hydroxide needed for complete neutralisation.

(i) Calculate the volume of sodium hydroxide that the student uses.

(ii) Calculate the pH of the resulting buffer solution, given that K_a = 9.3 x 10^-4.

Homework Equations

n = cV
HA <--> H⁺ + A^- (<--> indicating a dynamic equilibrium)

The Attempt at a Solution

For (i) I'm getting 4.5 cm³ which is in agreement with the mark scheme.

For (ii) the answer is 2.73, but I am not getting that, I'm getting 3.33.

Their answer implies that [A^-]/[HA] = 0.5, but how are they getting this?

 

[Borek]

If you neutralize 1/3 of the acid, solution contains one part of A^- and two parts of HA, hence [A^-]/[HA] = 1/2.

Actually exact answer is neither 2.73 nor 3.33

 

[FeDeX_LaTeX]

Thanks