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PH of solution from mixing four solutions

  1. Mar 15, 2009 #1
    1. The problem statement, all variables and given/known data
    What is the pH of the solution that results from mixing the following four solutions together:
    (i) 150 ml of 0.250M NaCl
    (ii) 300 ml of 0.200M HCl
    (iii) 100 ml of 0.050M HNO3
    (iv) 450ml of 0.200M NaOAc


    2. Relevant equations



    3. The attempt at a solution
    Firstly, NaCl does not contribute to the pH of the solution.
    HCl and HNO3 are both strong acids and contribute 0.065 mol of hydronium.
    There are also 0.09 mol of NaOAc (weak base) in the solution.
    The hydronium reacts with NaOAc, resulting in 0.025 mol of NaOAc in a 1L solution (0.025M NaOAc).

    Setting up an ICE table for acetate and acetic acid and using Kb for acetate is equal to 5.56x10-10 i worked out that the concentration of OH was 3.73x10-6.
    Then -log[OH] resulting in pOH of 5.43,
    pH of 8.57

    However, my answer is not in the choices that are given to me.
    choices:
    4.22
    4.33
    2.74
    4.98
    5.16

    can anyone help?
    thanks
     
  2. jcsd
  3. Mar 15, 2009 #2

    Borek

    User Avatar

    Staff: Mentor

    0.065M of hydronium
    0.09M of NaOAc

    Once they react, you have a solution containing both acetic acid and its conjugate base. Such a solution has a name and its pH is described by specific equation. ICE table is useless here.
     
  4. Mar 15, 2009 #3
    Thanks.
    I had completely forgotten about the acetic acid that forms.
     
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