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?