Calculating base ionization constant from a known pH and concentration

[doggbAT]

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

NH_3(aq) + H₂O_(l) <--> NH₄⁺_(aq) + OH^-_(aq)

pH = -log[H⁺]

K_b = [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

 

[symbolipoint]

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?

 

[doggbAT]

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) + NH_3(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 k_b = [NH₄⁺][OH^-]/[NH₃] and solved for k_b accordingly. However, when I calculated the percent difference between this and a known k_b value for ammonia, I got 199%. I don't think my titration skills are that bad.

 

[Borek]

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.