1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    M1*V1=M2*V2

    3. The attempt at a solution

    M1*V1=M2*V2
    (.2250M)*(20mL)=M2*(50mL)
    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

    symbolipoint

    User Avatar
    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

    Borek

    User Avatar

    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.

    --
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Titrations and pka's
  1. Pka and solubility (Replies: 3)

  2. PKa of Phenol (Replies: 4)

  3. Pl and pKa (Replies: 1)

  4. Organic pKa (Replies: 7)

Loading...