Ka , Dissociation Constant of a Weak Acid
Objective :
Determine Ka for an unknown weak acid using pH measurements.
Use the “LAB : HOW TO…” link from the class website if you need help with how to use balance,
Bunsen burner… and such.
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Procedure :
(1) Fill the first burette with ~0.200M unknown acid. Report the actual concentration.
(2) Fill the second burette with ~0.100M NaOH solution. Report the actual concentration.
(3) Prepare 5 solutions in large container by mixing
Solution # Volume of
Volume of
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
5
5.00
12.50
* The total volume should be 20.00mL, and you need to mix the solution well before pH probe is
inserted. Is your container big enough?
(4) Rinse out one of the burette and fill it with water. Add various mL of water to all 5 solutions to make
the total volume to be 20.00mL in each test tube. Mix well.
 Does pH change depending on the amount of water you put in? Explain.

Why do you think you are asked to add enough water to make 20.00mL?
(5) Measure the pH of solutions 1 to 5.
* The pH probe should be rinsed before, in between each solution, and at the end of the experiment.
* Let me know if the ‘storing solution’ for the pH probe is running low. DO NOT fill the container with
water.
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Prelab Questions:
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) Calculate moles of each. What is the limiting reactant?
(PL3) Calculate the moles of each left at the end of this reaction. Calculate the Molarities for each.
(What is the total volume?)
(PL4) Write a balanced equation for dissociation of HA(aq) in water.
(PL5) Write an expression for K for the equation above.
(PL6) Draw the appropriate ICE table leading to Ka. Fill in all the information.
Hint : Calculate [H3O+] at equilibrium from pH.
(PL7) Calculate Ka.
(PL8) What is the difference between K and Ka? Is the K from question above Ka? Explain.
Analysis :
(A1) Write the chemical equation (one that would lead to the Ka) and Ka expression.
(A2) Calculate Ka for each solution. At least for one solution, show your work clearly and completely so
anyone can follow what you have done to calculate Ka.
(A3) Calculate average Ka.
Postlab Questions :
(Q1) What would happen to the final answer if you forgot to add water to test tube #1 but used 20.00mL
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 :
Unknown number:
ONE Chemical equation that is associated with Ka :
Ka Expression :
Average Ka :
Abstract:
No more than 100 words stating why the lab was done, what was done, the major result(s), and
conclusion(s).
Reflection Statement:
One thing I might do differently next time is …… because…….