# Homework Help: PH of Buffer Solution

1. May 22, 2013

### FeDeX_LaTeX

1. The problem statement, all variables and given/known data
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 Ka = 9.3 x 10-4.

2. Relevant equations
n = cV
HA <--> H+ + A- (<--> indicating a dynamic equilibrium)

3. 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?

2. May 22, 2013

### Staff: Mentor

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

3. May 25, 2013

Thanks