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Homework Help: Acids and pH

  1. Jan 21, 2006 #1
    Question: Aspirin is a weak acid.

    (a) Calculate pH of 0.2M solution of aspirin at 25 degrees celsius (Ka = 3.0 x 10^-4 at 25 degrees celcius).

    (b) Determine the percent ionisation

    (c) Explain qualitatively the effect of adding 0.01M hydrochloric acid to the aspirin solution.

    (d) Calculate the pH of the resulting solution

    (e) Determine the percent ionisation of the aspirin under these conditions
  2. jcsd
  3. Jan 25, 2006 #2


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    well formula goes.. pH=-log[H+]
    am not sure what about aspirine...i am is Ka also meant as concentration of H+ ??
    if yes ...then

    pH=-log(3.0 x 10^-4)=3.5228

    and it is acid....but wouldnt say that weak
  4. Jan 25, 2006 #3


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    Sci0x : Please read the forum guidelines for getting help with schoolwork. See my signature.
  5. Jan 29, 2006 #4
    here is the most effective way with which u must proceed---

    1) the conc is given as .2 M and ka is given so find the [tex]H^+[/tex] conc using the formula [tex][H^+] = \sqrt{K_a * c}[/tex]
    2) percentage ionisation is given by the formula [tex]\sqrt{\frac{k_a}{c}}[/tex]
    3)Explain common ion effect
    4) due to addition of hydrochloric acid the conc of H+ will now be 0.01 + the conc of H+ u got in the 1st question
    5) repeate the 2 question but now with the new conc of H+ note use formula [tex]\frac{[H^+]}{c}[/tex]
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