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April 23, 2021, 08:51:02 pm

### AuthorTopic: Working out pKa value  (Read 1599 times) Tweet Share

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#### bebmtran

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##### Working out pKa value
« on: November 24, 2019, 11:50:34 am »
0
If a question asks to find the pKa of an acid, am I allowed to assume that $HA\rightleftharpoons H^++A^-$ and then use the equation
$K_a=\frac{[H^+][A^-]}{[HA]}$
Because of the dissociation equation the molar ratio of $[H^+]:[A^-]=1:1$, $[H^+]=[A^-]$,
$K_a=\frac{[H^+]^2}{[HA]}$
and then to calculate pKa
$pK_a=-log(K_a)$

I understand that the method I described isn't explicitly stated in the new NSW Chemistry Syllabus, so does this mean I cannot use this method? Or do I have to write out the chemical equation and use the ICE table to work out the concentration changes?

#### louisaaa01

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##### Re: Working out pKa value
« Reply #1 on: November 27, 2019, 10:15:40 am »
+2
If a question asks to find the pKa of an acid, am I allowed to assume that $HA\rightleftharpoons H^++A^-$ and then use the equation
$K_a=\frac{[H^+][A^-]}{[HA]}$
Because of the dissociation equation the molar ratio of $[H^+]:[A^-]=1:1$, $[H^+]=[A^-]$,
$K_a=\frac{[H^+]^2}{[HA]}$
and then to calculate pKa
$pK_a=-log(K_a)$

I understand that the method I described isn't explicitly stated in the new NSW Chemistry Syllabus, so does this mean I cannot use this method? Or do I have to write out the chemical equation and use the ICE table to work out the concentration changes?

Hi there!

This method is completely valid, and is in fact implicit in the Chemistry Syllabus, relying on a manipulation of the standard Ka expression with stoichiometry. The idea that [H+] = [A-] for a monoprotic acid (again based on stoichiometry) again comes into play when constructing an ICE table (as [H+] and [A-] change by the same amount) - so it isn't far removed at all from the Syllabus.

In an exam, just to be on the safe side and attain full marks, I would advise that you write a fully balanced equilibrium equation and the expression for Keq (based on the equation). An ICE table is a really nice, clear way to set out your ideas and determine the change in concentration, and so is advisable. However, if you wanted, you could get away with not using it so long as you don't get confused with your working out. The reason I'm saying this is that I've seen plenty of marking criterias from a range of 2019 Trial papers which, say, give a 4 mark question on Ka, and explicitly require you to write a balanced equation and the correct equilibrium expression to attain full marks (though as far as I know, I haven't seen any that require you to construct an ICE table).

Once you've established these, you can most certainly use the method you've outlined (ensuring you write every step with justification) to determine pKa.

I hope this helps. If you have any further questions, don't hesitate to ask!

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