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Homework Help: PH of Buffer Solution

  1. May 22, 2013 #1


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    Gold Member

    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. jcsd
  3. May 22, 2013 #2


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    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 :wink:
  4. May 25, 2013 #3


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