Determination of Titration Curves
Teacher Copy
Materials: (for 8 groups)
8 – burets
8 – buret clamps
8 – ring stands
8 – buret funnels
8 – LabQuests with pH probe
8 – 125 mL Erlenmeyer flasks
8 – 25 mL graduated cylinders
8 – 100 mL beakers
8 – 150 mL beakers for waste
8 – wash bottles
Costs:
LabQuest – Code: LABQ – Cost: $299
pH Sensor – Code: PH-BTA – Cost: $79
Solutions: (Please use either distilled or deionized water for all solutions)
Strong Acid - 0.1 M HCl: Add 1.7 mL of concentrated hydrochloric acid (12.1 M) to water
and dilute to 200 mL.
Strong Base – 0.1 M NaOH: Add 3.2 g of sodium hydroxide to 800 mL of water. Students
will use strong base for part 1 and 2. Store stock solution in a plastic bottle.
Weak Acid – 0.1 M CH3COOH: Add 1.1 mL of concentrated (glacial) acetic acid (17.4 M)
to water and dilute to 200 mL.
Weak Base – 0.1 M NH4OH: Add 2.7 mL of concentrated ammonium hydroxide (14.8 M)
to water and dilute to 400 mL.
Unknown Acid (option 1) – 0.1 M H2SO4: Add 1.1 mL of concentrated sulfuric acid (18 M)
to water and dilute to 200 mL.
Unknown Acid (option 2) – 0.1 M H3PO4: Add 1.4 mL of concentrated phosphoric acid
(14.8 M) to water and dilute to 200 mL.
Notes:
Depending on the amount of time you have to dedicate to this experiment you might
want to have two groups do each trial. This will allow students to compare data and see
that they should rely on their own data instead of what everyone else is collecting. This
also allows students to analyze a group that did the same trial as they did. For the
unknown solution, there should be two or three equivalence points based on the
number of acidic protons in the unknown that you choose. Have the students use
sodium hydroxide as the base for this titration and make sure that they clean the buret
between each trial.
Prelab Questions:
1. What is the Bronsted-Lowry definition of an acid? What is the Bronsted-Lowry
definition of a base?
Bronsted-Lowry acids are proton (H+) donors
Bronsted-Lowry bases are proton (H+) acceptors
2. What is the difference between a strong acid and a weak acid?
Strong acids completely dissociate in water and have very low pH values and
weak acids only partially dissociate in water and have pH values closer to 7.
3. Write a balanced molecular reaction between hydrochloric acid and sodium hydroxide.
HCl + NaOH → NaCl + H2O
4. When diluting an acid in water is the water added to the acid or the acid added to the
water? Why?
Always add acid to water. This will help prevent any violent reaction that may
splash on your skin, clothing, or in your eyes.
Discussion Questions:
1. Why does the pH of the solution not change dramatically when base in initially added?
When base is initially added to the solution, the pH should not change
dramatically because there is still more protons in solution than hydroxide ions.
2. Why do the shapes of each of the following trials vary?
Based on the strength of the acid or the base the shape of the titration curve will
vary. However they should all have the same basic structure of a low initial pH
with a semi-flat line as base is added. There should be a dramatic change in pH
at some point in the experiment with another semi-flat line a much higher pH
value.
3. What is the pH of the equivalence point of each of the following trials?
Answers will vary
4. According to the molecular reaction for Trial 1 why should the pH of the equivalence
point be 7? What is the percent error for the equivalence point for Trial 1 according to
your data?
In theory the pH should be 7. Percent error calculations will vary due to student
data.
5. For Trial 1 why is there such a dramatic shift in pH near the equivalence point?
Near the equivalence point there is a great change in the concentration of
protons and as more base is added more hydroxide ions in solution than protons.