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PH and KA calculations

Discussion in 'DAT Discussions' started by Awuah29, Apr 12, 2007.

  1. Awuah29

    Awuah29 Christian predent
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    How do you guys set these problems up ?

    14. What is the pH of a solution, which is 0.5 M in CH3COOH (HOAc) and 0.5 M in NaCH3COO (NaOAc)?
    CH3COOH: Ka = 1.8 x 10-5

    a) 2.9
    b) 4.7
    c) 5.9
    d) 7.0
    e) 8.9


    What is the Ka of the weak acid phenylacetic acid, C6H5CH2COOH, if 2.1 % of the acid dissociates in a 0.12 M solution ?

    a) 5.2 x 10-5
    b) 3.7 x 10-5
    c) 2.1 x 10-2
    d) 1.8 x 10-1
    e) 3.7 x 101

    What is the pH of a 0.020 M citric acid , H3C6H8O7, solution ?
    citric acid: Ka = 7.5 x 10-4

    a) 1.6
    b) 2.2
    c) 3.1
    d) 3.7
    e) 4.4
     
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  3. BodybldgDoc

    BodybldgDoc Guest

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    Use this formula,
    pH=pka + log(conj base/weak acid)

    NaCH3COO=base and CH3COOH=acid
    so for #14, you would have pH= 4.74 + (0.5M/0.5M) = 4.74
    (you get the pka by taking the log of 1.8E-5=4.74)

    do the same thing for the rest of the problems.
     
  4. Awuah29

    Awuah29 Christian predent
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    so basically, you can estimate that a log of 1.8E- 5 will be around. 4?

    do we have to know logs?
     
  5. BodybldgDoc

    BodybldgDoc Guest

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    yeah you do have to be able to estimate the log of something. For example Ka=1.8E-5 then the pKa=5-log1.8 But Since 1.8 is a small number, its log will also be small and the answer will be closer to 5 than to 4. The kaplan blue book shows you how to make the estimation really well.
     

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