1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can someone explain?

  1. Nov 1, 2008 #1

    a.a

    User Avatar

    1. The problem statement, all variables and given/known data

    The approximate pH of a 1 M solution of NH4Br [Kb(NH3) = 1.8 x 10-5] is___

    2. Relevant equations



    3. The attempt at a solution

    Ka(NH4) = 10^-14 / 1.8 x 10^-5 =
    =5.5555 x 10^-10

    therefore, 5.5555 x 10^-10 = [NH3][H+]/[NH4+]

    Since 1 H+ forms for every NH3 that forms and that [NH3] + [NH4+] = 1,
    5.5555 x 10^-10 = [H+][H+]/(1-[H+])
    Since Ka is small, 1-[H+] approximately = 1
    [H+] = 2.357 x 10^-5
    ph = 4.63

    where did the 10^-14 come from and why are we saying this:
    5.5555 x 10^-10 = [NH3][H+]/[NH4+]
     
  2. jcsd
  3. Nov 1, 2008 #2

    Borek

    User Avatar

    Staff: Mentor

    10-14 is Kw.

    The other part is about Bronsted-Lowry theory. NH4+ is a Bronsted-Lowry acid. Expression that you have listed is its dissociation constant.
     
  4. Nov 3, 2008 #3

    a.a

    User Avatar

    what exactlly is pKa and pKb?
     
  5. Nov 3, 2008 #4

    Borek

    User Avatar

    Staff: Mentor

    pKa = -log (Ka)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?