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Calculating base ionization constant from a known pH and concentration

  • Thread starter doggbAT
  • Start date
  • #1
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Homework Statement


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


Homework Equations


NH3(aq) + H2O(l) <--> NH4+(aq) + OH-(aq)

pH = -log[H+]

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

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:

Answers and Replies

  • #2
symbolipoint
Homework Helper
Education Advisor
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This is just not sufficient.

I have completed the experiment, and have the following:

Initial concentration of NH3: 3.708mol/L
Initial pH of NH3: 10.26
How about a pH titration as additional information?
 
  • #3
7
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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:
  • #4
Borek
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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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