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Homework Help: Calculating base ionization constant from a known pH and concentration

  1. May 13, 2009 #1
    1. The problem statement, all variables and given/known data
    In this experiment you will find the ionization constant for ammonia, a weak base.

    I have completed the experiment, and have the following:

    Initial concentration of NH3: 3.708mol/L
    Initial pH of NH3: 10.26

    2. Relevant equations
    NH3(aq) + H2O(l) <--> NH4+(aq) + OH-(aq)

    pH = -log[H+]

    Kb = [NH4+][OH-]/[NH3]

    3. The attempt at a solution
    I previously had no idea what to do, but now looking at the ICE table I have...

    NH3 + H2O <--> NH4 + OH
    I 3.708 - 0 0
    C -x - +x +x
    E 3.708-x x x

    Correct me if I'm wrong, but...
    Would the E value of NH3 not be 0?
    And wouldn't the E values of NH4 and OH be 3.708?

    Thanks, doggbAT
    Last edited: May 13, 2009
  2. jcsd
  3. May 13, 2009 #2


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    Homework Helper
    Education Advisor
    Gold Member

    This is just not sufficient.

    How about a pH titration as additional information?
  4. May 13, 2009 #3
    Okay I figured out that the information I gave was wrong.

    I measured the pH of the NH3 in its source bottle to be 10.26
    I then titrated it with 9.275mL of .0100mol/L HCl.
    I calculated the amount of HCl to be 9.275x10-5mol.
    Based on the equation HCl(aq) + NH3(aq) <--> NH4+(aq) + CL-(aq), I assumed that the amount of HCl is equal to the amount of NH3.
    Therefore, the amount of NH3 is 9.275x10-5mol.
    I then used that amount to determine the concentration to be 3.708mol/L

    I do not understand when this concentration of NH3 is true. I went under the assumption that it was the concentration of the original bottle of NH3. I used the pH of the original bottle to determine the [H+], then the [OH-]. I made an ICE table, and subbed the known [OH-] value into it. I determined that kb = [NH4+][OH-]/[NH3] and solved for kb accordingly. However, when I calculated the percent difference between this and a known kb value for ammonia, I got 199%. I don't think my titration skills are that bad.
    Last edited: May 14, 2009
  5. May 14, 2009 #4


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    Staff: Mentor

    My bet is that your initial pH is wrong. pH 10.26 gives ammonia concentration in the range of 10-3M. Molar concentrations mean pH close to 12.
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