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To what volume should 100ml of any weak acid (HA) with a concentration of 0.2M be diluted to double the percentage ionization?

  • chemistry - ,

    I think you are looking at this scenario.
    Say HA = 0.2 and (H^+) = 0.001M, then
    %ion = (0.001/0.2)*100 = 0.5%.
    Now if you dilute HA from 100 to 200 mL, (HA) is now 0.1M and %ion, if (H^+) = 0.001, is (0.001/0.1)*100 = 1.0% which is double the initial.
    HOWEVER, I'm not convinced that will happen for any and all weak acids.
    For example, look at acetic acid with Ka = 1.8E-5. If we use 0.2M HAc, then
    1.8E-5 = (x)(x)/0.2 and x = (H^+) = 0.00190 and % ion = 0.949%
    Now we dilutethe acid from 0.2 to 0.1 but (H^+) changes. Instead, it is
    1.8E-5 = (x)(x)/(0.1) = 0.00134 which is 1.34% and that isn't double 0.949%.
    I may have misinterpreted the question; if so let me know.

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