Average the pH values in Data Table 2 for Trials 1 and 2 and record in pH value (Average) in Data Table 2. The average pH at half-equivalence point is equal to the pKa of the unknown weak acid. Record this value in Data Table 3 next to pKa.
Determine the Ka of the unknown weak acid using the following equation: Ka = 10-pKa
Record the Ka of the unknown weak acid in Data Table 3.
Use the accepted values for Ka and pKa for a variety of acids in Table 2 to determine the identity of the unknown weak acid. Record the identity of the unknown weak acid in Data Table 3. Drops NaOH Added pH Value Trial 1 pH Value Trial 2 pH Value (Average)
Half-Equivalence Point
Equivalence Point
0 2 2 2
10 4 3 3.5
20 4 4 4
30 4.5 4 4.25
40 5 4 4.5
50 5 5 5
60 5 5 5
70 5 5 5
80 5 5 5
90 5 5 5
100 5 5 5
110 5 6 5.5
120 6 6 6
Volume of Unknown Weak Acid (mL) 4mL
Total Number of Drops Required to Reach Equivalence Point trial 1: 146; Trial 2: 135
Average Number of Drops 140.5
1. Using the data collected in Data Table 2, create a graph of "Drops of NaOH added" vs "Average pH" where "drops of NaOH added" is on the x-axis and "pH" is on the y-axis. Upload an image of the graph into Graph 1.
2. 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?
3. Discuss possible causes of error in the experimental procedure. Why do you think there was error in the experimentally determined pKa and Ka values of the unknown weak acid, in comparison to the values presented in Table 2?
4. Do you think using a pH meter instead of pH indicator strips would have created a larger or smaller percent error? Explain your answer.
I think the meter would have caused a smaller percent error just because it is being read from the indicator and not from the eyes of the person doing the experiment so it would allow for a more accurate reading
5. Why was phenolphthalein a good indicator to use for determining the equivalence point between the unknown weak acid and strong base?
because it changes to a pink color once a reaction has occurred amongst an acid and a base. Colorless in an acid but pink in a basic solution so once it changes pink that shows the solution has went from acidic to basic.