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Chemistry - Common Ion Effect

  1. Nov 15, 2007 #1
    What is the pH change of a 0.270 M solution of citric acid (pKa = 4.77) if enough sodium citrate is added to make the final concentration of the common citrate ion 0.145 M?

    Okay so first I did an ice-table using the equation:
    citric acid + water <--> hydronium + citric ion
    Using an inital concentration of 0.270 for citric acid and then just solving for x using the Ka value and I got pH = 2.67 (which is the initial pH).
    Now I'm having a problem with how to find the final pH. I think you're supposed to set up another ice table but I'm not too sure what values are supposed to be my initial concentrations. Any help would be greatly appreciated!
  2. jcsd
  3. Nov 16, 2007 #2
    HA + H2O <--> H3O+ + A-

    [HA] = 0.270 - x
    [H3O+] = x
    [A-] = 0.145

    [H3O+]*[A-]/[HA] = Ka = 10^(-4.77)

    --> x*0.145/(0.270 - x) = 10^(-4.77)

    --> x = 3.16*10^(-5) --> pH = 4.50
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