Calculating base ionization constant from a known pH and concentration

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Discussion Overview

The discussion revolves around calculating the base ionization constant (Kb) for ammonia based on experimental data, including initial concentration and pH measurements. Participants explore the relationship between pH, concentration, and ionization in the context of weak bases, specifically ammonia.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents initial data from an experiment involving ammonia, including its concentration and pH, and sets up an ICE table to analyze the equilibrium.
  • Another participant suggests that the initial information provided is insufficient and proposes a pH titration as a potential additional method for analysis.
  • A later reply indicates that the initial pH measurement may have been incorrect, suggesting that a pH of 10.26 corresponds to a much lower concentration of ammonia than initially stated.
  • The same participant expresses confusion regarding the validity of the ammonia concentration derived from the titration and questions the accuracy of their calculations, particularly when comparing their results to known Kb values for ammonia.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the accuracy of the initial pH measurement or the resulting calculations. There are competing views regarding the reliability of the experimental data and the implications for determining Kb.

Contextual Notes

There are unresolved assumptions regarding the accuracy of the pH measurement and the concentration of ammonia, as well as potential errors in titration methodology. The discussion reflects uncertainty about the relationship between the experimental conditions and the theoretical values.

doggbAT
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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:
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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?
 
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:
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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