ONLY FOR MISS HASHMI

Determination of Ka for a Weak Acid

Hands-On Labs, Inc.
Version 42-0151-00-02

Lab Report Assistant

This document is not meant to be a substitute for a formal laboratory report. The Lab Report Assistant is simply a summary of the experiment’s questions, diagrams if needed, and data tables that should be addressed in a formal lab report. The intent is to facilitate students’ writing of lab reports by providing this information in an editable file which can be sent to an instructor.

Exercise 1: Determination of Ka

Data Table 1. Determination of Equivalence Point.

	Trial 1	Trial 2
Volume of Unknown Weak Acid	4 mL	4 mL
Total Number of Drops Required to Reach Equivalence Point
Average Number of Drops

Data Table 2. Titration Curve Values.

Drops NaOH Added	pH Value Trial 1	pH Value Trial 2	pH Value (Average)
Half-Equivalent Point = (46 Drops)
Equivalent Point = (93 Drops)

10
20
30
40
50
60
70
80
90
100
110
120

Data Table 3. Determination of Unknown.

pKa of Unknown Weak Acid:
Ka of Unknown Weak Acid:
Unknown Weak Acid Identity:
% Error pKa:
% Error Ka:

Questions

Using the data collected in Data Table 2, create a graph of “Drops of NaOH added” vs “pH” where “drops of NaOH added” is on the x-axis and “pH” is on the y-axis.
pH indicator strips work by changing color in the presence of solutions with varied pH values. Thinking about your procedure steps and results in Part 1, why do you think the pH of the unknown weak acid was not determined with pH indicator strips until Part 2?
Discuss possible causes of error in the experimental procedure. Why do you think there was a percent of error in the pKa and Ka values of the unknown weak acid, in comparison to the values presented in Table 2?
Do you think using a pH meter instead of pH indicator strips would have created a larger or smaller percent error? Explain your answer.
Why was phenolphthalein a good indicator to use for determining the equivalence point between the unknown weak acid and strong base?