Ka , Dissociation Constant of a Weak Acid
Objective :
Determine Ka for an unknown weak acid using pH measurements.
Prelab: In addition to writing your procedure and any data tables into your
notebook, complete the following.
10.00mL of 1.000M HA (a weak acid) was mixed with 5.00mL of 0.500M NaOH.
Then the mixture was diluted to 50.0.mL with distilled water. The pH of the
solution read 5.00.
(PL1) What is the chemical equation between HA and NaOH.
(PL2) What is the limiting reactant?
(PL3) What is [HA] and [OH-] left at the end of this reaction?
(PL4) Write a balanced equation for dissociation of HA in water.
(PL5) Write an expression for Ka for HA.
(PL6) Draw the appropriate ICE table. Fill in all the information.
Hint : Calculate [H3O+] at equilibrium from pH.
(PL7) Calculate Ka.
Procedure:
(1) Fill the first buret with the ~2.00M unknown acid.
(2) Fill the second buret with the 0.1005 M NaOH solution.
(3) Prepare 4 solutions in large test tubes by mixing
Solution #
Volume of Unknown M
Volume of 0.1005M
Unknown acid (mL)
NaOH (mL)
1
5.00
2.50
2
5.00
5.00
3
5.00
7.50
4
5.00
10.00
(4) Rinse out one of the buret and fill it with water. Add various mL of water to all
4 solutions to make the total volume to be 15.00 mL in each test tube. Mix
well.
(5) Measure the pH of solutions 1 to 4.
Postlab: In addition to reporting your data results, conclusions, learning,
problems, and errors, complete the following.
Calculations:
(C1) Write the chemical equation (one that would lead to the Ka) and Ka
expression.
(C2) Draw 4 ICE tables with all the information filled in.
(C3) Calculate Ka for each solution.
(C4) Calculate average Ka.
Questions:
(Q1) What would happen to the final answer if you forgot to add water to test
tube #1 but used 15.00mL as your total volume in your calculation?
(Q2) The calculations are done assuming that OH- added is reacting completely.
What would happen to the final result if not all OH- was used up?
Hint: WHAT is the final result? What would happen to the VALUE? Why?
Summary Data Table(s):
Unknown number
Re-draw I.C.E. table for solution #1
Average Ka