Turn on thread page Beta

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

    • Thread Starter
    Offline

    10
    ReputationRep:
    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 ?
    • Community Assistant
    • Study Helper
    Offline

    17
    ReputationRep:
    Community Assistant
    Study Helper
    (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.
    Offline

    15
    ReputationRep:
    (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.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: February 1, 2017

University open days

  1. University of Bradford
    University-wide Postgraduate
    Wed, 25 Jul '18
  2. University of Buckingham
    Psychology Taster Tutorial Undergraduate
    Wed, 25 Jul '18
  3. Bournemouth University
    Clearing Campus Visit Undergraduate
    Wed, 1 Aug '18
Poll
How are you feeling in the run-up to Results Day 2018?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.