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When to use Kw and Ka

  1. Sep 19, 2015 #1
    So I've learnt that in strong acids that dissociate completely, the concentration of H+ is the same as the concentration of the initial solution. So 1M of a strong acid will create 1M of H+, meaning the pH is 0.

    I've also learnt that in weak acids, the whole thing doesn't dissociate. I have to use the acid dissociation constant to calculate the concentration of H+.

    But in some parts of my textbook, it also says I can use Kw to calculate the pH of acids, eg. if a solution has a concentration of 10^-4 H+ ions, then the pH is 4.

    But wouldn't I need to first find the acid dissociation constant of that solution?

    In other words, when can I use Kw, and when do I have to calculate Ka? Can Kw be used only for water, or does it apply to some acids as well?

    Thanks a lot for your help.
  2. jcsd
  3. Sep 19, 2015 #2


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    Staff: Mentor

    How does it differ from the

    Kw starts to be important when the concentration of acid (be it strong or weak) is so low, you can't ignore H+ from the water autodissociation.

    What is pH of a 10-8 M HCl?
  4. Sep 19, 2015 #3
    Okay so concentration is different from whether an acid is strong or weak?

    Meaning if you have a diluted acid, then you use Kw, but if you have a concentrated acid, whether it's strong or weak, you have to use Ka to calculate the pH?
  5. Sep 19, 2015 #4


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    Staff: Mentor

    More or less.

    For a very weak acid it may happen that even if it is not highly diluted you need to take Kw into account.

    In general, as I already wrote, it depends on whether the H+ from the water autodissociation can be ignored or not. It can be ignored when it is orders of magnitude lower than the concentration of H+ from the dissociation of acid (note: doesn't matter whether the acid is strong or weak, what matters is how much H+ is produced during dissociation).
  6. Sep 20, 2015 #5
    Okay thanks.

    Also, in a weak acid, with the value of Ka, is it possible to find Kb? Can the Kw be used to do this for all acids?

    Eg. if my value of Ka is 10^-4, is Kb always going to be 10^-10? For every single acid?

    And if the H+ concentration is 10^-4, then is the base concentration always going to be 10^-10 as well?

    Thanks for your help.
  7. Sep 20, 2015 #6


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    Staff: Mentor

    Easy to prove with a simple math. Just beware - Ka and Kb must be those of the conjugated pair of acid and base, and in the latter case if the H+ is 10-4 M it is concentration of OH- that is 10-10, not necessarily a concentration of a "base" (whatever it is intended to be).
  8. Sep 20, 2015 #7
    Okay thanks
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