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

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owjay
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Hi
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
then
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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TSR Jessica
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Sorry you've not had any responses about this. Are you sure you've posted in the right place? Here's a link to our subject forum which should help get you more responses if you post there.
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