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Homework Help: Titrations and pka's

  1. Nov 10, 2009 #1
    1. The problem statement, all variables and given/known data

    Weak acid analyte with strong base titrant.

    What is the pKa of the analyte in this titration to the nearest 0.5?

    Given: titrant molarity is .2250 M ; 50 mL of analyte present ; 20 mL of titrant ; initial pH of 3.0

    2. Relevant equations


    3. The attempt at a solution

    M2 = .09M

    Knowing the molarity, I made a table and got:

    Ka = x^2/(.09-x), where x is the concentration of H^+ ions.

    -log(x) = 3.0
    x = .001

    Ka = (.001)^2/.09 = 1.11e-5

    pKa = -log(1.11e-5) = 4.95 ----------> Since to nearest 0.5, pKa = 5.0

    This last part is where I'm confused. I submitted this answer for my homework and it said it was incorrect. Is this not what they meant by nearest 0.5? Any help is much appreciated.
    Last edited: Nov 10, 2009
  2. jcsd
  3. Nov 10, 2009 #2


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    Homework Helper
    Education Advisor
    Gold Member

    K = [tex]\frac{(H)(A)}{HA}[/tex] , ignoring any charges. Monofunctional weak acid, HA.

    -log(K) = -(log(H) + log(A/HA)

    pK = pH + log(A/HA)

    What happens at half-neutralization of the acid? [A]/[HA] = 1 (Do you understand this?) The concentration of unneutralized acid and neutralized acid is the same, equal so their ratio is 1.

    pK = pH + log(1) = pH + 0 = pH.
    pK=pH at half-neutralization.
  4. Nov 11, 2009 #3


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    Staff: Mentor

    Is it all that is given? Does it mean 20 mL used to titrate 50 mL? If so, question reads: what is pKa of the acid if 0.09M solution has pH 3.0, and 5.0 seems to be OK.

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