Titration of a Weak Base

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Page ID: 70841

Thomas Wenzel
Analytical Sciences Digital Library

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In-class Problem Set #2

1. Starting with 30 mL of a solution that is 0.1 M in butylamine, calculate the original pH, and then the pH as 5 mL increments of 0.1 M hydrochloric acid are added. Continue the series of calculations until 40 mL of acid have been added. Plot the data (pH on the y-axis, volume of added acid on the x-axis).

Has 99.9% of the butylamine been titrated at the equivalence point?

What is the initial pH of the solution?

It should take the groups a few minutes to calculate the initial pH of the system. Point out after that this is a solution of a weak base so the initial pH being basic is not a surprise.

What happens when we add 5 mL of 0.1 M HCl?

Allow the students a few minutes to think about what will happen then spend some time discussing the neutralization and conversion of butyl amine to butyl ammonium, leading to the formation of a buffer. Talk about how the concentration of butylamine will change with each addition of HCl, but if we use moles we can ignore the change in volume (consider the form of the Henderson-Hasselbalch and how each component of the buffer is in the same volume so a mole ratio is the same as the ratio of molarities).

Sometimes students ask about the amount of butyl amine that reacted to produce butyl ammonium in the initial solution (0.002 moles of the 0.030 moles in solution) and whether they should start with 0.030 or 0.028 moles of butyl amine. Or you may wish to raise this as a question for the class.

Discuss the fact that 0.002 moles of hydroxide ion (a strong base) is produced in the initial solution and that this reacts first with 0.002 moles of the HCl. Once the hydroxide is used up, the butyl amine begins to react.

What will happen after an additional 5 mL of 0.1 M HCl is added?

Have students make a table to keep track of the moles of each species present at each step up to 30 ml of added titrant. It may take the students about fifteen minutes to correctly set up the table and decide how to fill in each column.

What happens after the addition of 30 mL of 0.1 M HCl? How do you find the pH at that point?

Students should recognize that once all of the butylamine has been used up there is no longer a buffer. Talk about the equivalence point and various methods of indicating the progress of a titration. Students may be tempted to calculate the pH by evaluating how much of the excess strong acid has been added, but should recognize that only butylammonium is present and so the question simply becomes a weak acid problem. It may take them about ten minutes to realize this and calculate the pH.

What is the pH when 35 and 40 ml of titrant has been added?

Most groups readily see that there is now extra strong acid that will be most important in determining the pH.

What would a plot of pH vs. mL of acid added look like?

Have the students make a plot of the titration curve and point out distinguishing features such as the equivalence point, buffer region, and the inflection point. Discuss how the plot would look if the base had a pK_a of 8. How would it look if the pK_a was 6?

Has 99.9% of the butylamine been titrated at the equivalence point?

Students may have some trouble recognizing that calculating the x value for the K_a expression at the equivalence point can provide them with information regarding how much butylamine is in the system at the end of the titration.