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Acids and Bases - Buffers

  1. Jun 15, 2008 #1
    Hi there

    I was wondering if anyone could help me with this buffers question ...
    For which of these liquids is the pH likely to change least if a splash of concentrated sulphuric acid is added? Justify your answer:

    a) 0.1M hydrochloric acid solution
    b) 0.08M ethanoic acid/0.02M potassium ethanoate mixture
    c) 0.8M ammonia/0.2M potassium ethanoate mixture
    d) pure water

    a is a strong acid and therefore it will have no buffering capacity ... and pure water (d) has no buffering capacity either ... Which leaves b and c...

    The answer is (c) but how do you come to this answer?

    There's a similar question:
    "Which of these liquids will have the least change in pH if a sodium hydroxide pellet were to drop into a container of the liquid? Justify your answer:

    a) pure water
    b) 0.01 M ammonium chloride/0.01M ammonia mixture
    c) 0.05M ammonium chloride/0.05 M ammonia mixture
    d) 0.01M ammonium chloride/0.1M ammonia mixture

    (Correct answer is c)


    Any help will be much appreciated :)
     
  2. jcsd
  3. Jun 15, 2008 #2

    Borek

    User Avatar

    Staff: Mentor

  4. Jun 15, 2008 #3
    Q1.
    (b)-0.08M ethanoic acid/0.02M potassium ethanoate mixture = 0.1M
    (c)-0.8M ammonia/0.2M potassium ethanoate mixture = 1M

    Therefore the ammonia/potassium ethanoate mixture has the largest buffering capacity ...?

    But then ...
    Q2.
    (b) 0.01 M ammonium chloride/0.01M ammonia mixture = 0.02M
    (c) 0.05M ammonium chloride/0.05 M ammonia mixture = 0.1M
    (d) 0.01M ammonium chloride/0.1M ammonia mixture = 0.11M

    Wouldn't (d) have the largest buffering capacity because it has the highest concentration ...? Or should both acid and base concentrations be equal (d is more alkali as ammonia contributes much more than the ammonium chloride) so c is the best answer because there's an even amount of concentration from the conj. acid and base ...?
     
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