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Chemistry pH question

  1. Oct 27, 2011 #1
    1. The problem statement, all variables and given/known data

    how would you make a 5pH solution from citric acid(1L)



    2. Relevant equations

    pH= log(H+)


    3. The attempt at a solution

    So I will try to work backwards from the [H+]

    antilog(-5) = 10^-5 M H+ needed

    this number can be divided by 3, because citric acid is triprotic

    3.33*10^-6 moles * 210.14 g/mol = 7*10^-4 g brought to volume with 1 liter
     
  2. jcsd
  3. Oct 28, 2011 #2

    Borek

    User Avatar

    Staff: Mentor

    That would be way too simple. Citric acid is a weak one, with pKa values of 3.13, 4.76 and 6.40, so at pH 5.0 none of the protons is dissociated 100%. That means your assumption is faulty, and concentration you calculated - while not far from the correct answer - is too low.

    To be honest, I can't think of a simple and accurate approach to solving the question on paper. It is not a problem using - although it requires some trial and error to get the correct concentration, as program calculates pH for a given concentration, but it doesn't work the other way around.

    Are you sure the problem is about using citric acid and not about preparing citric buffer?
     
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