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Working out the PH of a weak Acid with 1:2 Ratio Watch

    • Thread Starter

    Just wondering, If you need to work out a weak acids PH, but it is not a simple 1:1 Ratio, how would you do it?

    HA -------> H+ + A-
    For this you simply have Ka = [H+]squared divided by [HA] or [H+][A-] divided by [HA] to give you the concentration of H+
    Then PH = - log[H+]

    But if you now had;
    2HA -------> H+ + HA2-
    (made this up - really just to show difference in ratio)

    When would you work out the ratio, after you worked out the concentration of [H+] using Ka as in the first example, then simply divide by two (ratio 2:1) to take into account of the ratio?
    Or would you substitute the coefficencies as the powers in the actual Ka formula
    Ka = [H+][HA2-] divided by [HA]squared
    or if this was the formulae
    H2A----> 2H + A
    Ka = [H]squared times by [A] divided by [H2A] ???

    Not sure if part of A2 AQA Chemistry but still like to know the answer.

    Thanks Oliver John
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