Calculating base ionization constant from a known pH and concentration

In summary, the conversation discusses an experiment to find the ionization constant for ammonia, a weak base. The initial concentration of NH3 is 3.708mol/L and the initial pH is 10.26. Additional information about a pH titration is also provided. However, there is uncertainty about the accuracy of the initial pH measurement and its impact on the calculations.
  • #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:
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  • #2
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
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.
 
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  • #4
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.
 

What is the base ionization constant?

The base ionization constant, also known as the base dissociation constant, is a measure of the strength of a base. It is represented by the symbol Kb and is equal to the ratio of the concentration of hydroxide ions (OH-) to the concentration of the undissociated base.

What is the relationship between pH and base ionization constant?

pH and base ionization constant are inversely related. As the pH increases, the concentration of hydroxide ions increases and the concentration of the undissociated base decreases, resulting in a higher base ionization constant. Similarly, as the pH decreases, the base ionization constant decreases.

How do you calculate base ionization constant from a known pH and concentration?

To calculate the base ionization constant from a known pH and concentration, you can use the equation Kb = [OH-]^2/[B], where [OH-] represents the concentration of hydroxide ions and [B] represents the concentration of the undissociated base. Both of these values can be determined from the given pH and concentration.

What are the units of base ionization constant?

The units of base ionization constant depend on the units of concentration used. If the concentration is given in moles per liter (M), then the units of Kb will be M^-1. If the concentration is given in grams per liter (g/L), then the units of Kb will be (g/L)^-1.

What factors can affect the accuracy of calculating base ionization constant from a known pH and concentration?

The accuracy of calculating base ionization constant from a known pH and concentration can be affected by factors such as temperature, ionic strength, and the presence of other ions or compounds that may interact with the base. It is important to ensure that the conditions of the solution are consistent and controlled for accurate calculations.

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