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

1. 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
2. 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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