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# How do I know which equation to use in acid base equilibrium?? watch

1. 1) Ka= [H]2/[HA]
Or
2) pH=pKa+log10(A-/HA)

For 2 I could covert pKa to Ka and the equation to equal to Ka (place pH on the other side)
Is this possible
Don't worry it's hard to understand^
How do I use the each equation correctly when a question asks for it ?
Is 2) just for buffer solutions only ?
2. (Original post by Hazel99)
1) Ka= [H]2/[HA]
Or
2) pH=pKa+log10(A-/HA)

For 2 I could covert pKa to Ka and the equation to equal to Ka (place pH on the other side)
Is this possible
Don't worry it's hard to understand^
How do I use the each equation correctly when a question asks for it ?
Is 2) just for buffer solutions only ?
It's mostly a question of style.

Unless an exam question specifically requires you to use one or the other I would always plump for the ka equation simply because you will not make a mistake deriving it, while there is always a chance with the HH equation (the log form) that you will get the fraction or the sign incorrect.
3. (Original post by Hazel99)
1) Ka= [H]2/[HA]
Or
2) pH=pKa+log10(A-/HA)

For 2 I could covert pKa to Ka and the equation to equal to Ka (place pH on the other side)
Is this possible
Don't worry it's hard to understand^
How do I use the each equation correctly when a question asks for it ?
Is 2) just for buffer solutions only ?
I agree with the above. I find it easiest to derive everything from Ka = [H+][A-]/HA. If you do the algebra step by step you're less likely to forget, for example:

The square root sign if rearranging to find simply the pH of weak acids ---- H+ = √Ka * Ha

(assuming [H+] and [A-] are the same. So you use [H+]^2)

The Henderson Hasselbalch equation is generally mostly used for buffer solutions. At higher levels you can use it to calculate the isoelectric point of proteins. At A level you'll only use it for buffers. If you know how to take logs and inverse logs, you should have no problems.

The equations aren't hard to use, the tricky part sometimes is extracting the relevant information from the question. Requires a strong understanding of the theory behind Acids and Bases.

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