1. Limited time only! Sign up for a free 30min personal 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!

Figuring pH of .20M NH3 and .35M NH4Cl

  1. Oct 31, 2012 #1
    1. The problem statement, all variables and given/known data
    Calculate the pH of a solution that is .20M NH3 and .35M NH4Cl with a Kb(NH3)=1.8x10^-5


    3. The attempt at a solution
    So first calculated the M of OH- produced by the NH3,
    [OH-]=√((1.8x10^(-5))x.20) = 1.9x10^-3

    Since NH4Cl will dissociate in H2O it will leave NH4+, NH4+ + H2O ---> NH3 + H30+
    Ka would be [1x10^(-14)]/[1.8x10^(-5)] = 5.6x10^(-10) giving us a rather insignificant amount of hydronium produced compared to the amount of OH- from the NH3 but lets figure it none the less,

    [H3O] = √(5.6x10^(-10))x.35 = 1.4x10^(-5)
    Since the hydronium will react with the OH- subtracting the two gives us 1.88x10^-3M OH-
    giving us a pOH of 2.73, or pH of 11.27

    The answer is pH 9.01 so I am waaaaay off.
    This is new stuff for my brain and that's the excuse I'm sticking with :)
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Oct 31, 2012 #2

    Borek

    User Avatar

    Staff: Mentor

    This is a buffer solution.
     
  4. Oct 31, 2012 #3
    I was breaking it down a little further than the Henderson–Hasselbalch equation so I can get a better conceptual understanding.

    The following is correct for that equation is it not?

    pH = pKa + log (base/acid)
    so pH = 4.74 + log [1.9x10^(-3)]/[5.6x10^-10]

    I still get 11.27pH, yikes!

    I must be screwing up on the molarity, do you see where I put my marbles on this thing Borek?
     
  5. Oct 31, 2012 #4

    Borek

    User Avatar

    Staff: Mentor

    Your problem is with the identification of the base and its conjugate acid. These are not OH- and H+.
     
  6. Oct 31, 2012 #5
    Blamo, Marbles found! :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Figuring pH of .20M NH3 and .35M NH4Cl
  1. Percentage of NH3? (Replies: 1)

  2. Kp of NH4Cl reaction (Replies: 5)

Loading...