Dominic Held
Micro Scale Titration
Overview:
In this experiment, the method of titration will be used to determine the gram
molecular weight (GMW) of citric acid by titration with NaOH. Using the experimental
GMW, the normality of citric acid will be calculated. Phenolphthalein will be used to
indicate when the titration is complete. With just 1 to 2 drops of phenolphthalein, the
color of the solution will be colorless when it is acidic, and become pink when the
solution is neutralized.
This titration will be performed on a micro scale. Instead of measuring the
volume of acid and bases used, drops from a transfer pipette will be counted.
Objectives:
Students will prepare a 0.2 molar solution from 1.0 molar solution of NaOH that
will be supplied to them. Students will gain experience with precision when working
with the small amount of solution that is used. They will also be introduced to the
method of titrations. Students will then calculate the gram molecular weight of citric
acid using their data.
Materials:
24 well reaction plates
Transfer pipettes
0.2 M NaOH
Coffee stirrers
Phenolphthalein
Procedure:
Groups of 2 will be assigned.
1. Students must wear safety goggles at all times.
2. Using the 1.0 M NaOH solution that is given, make a small sample of a 0.2 M
solution of NaOH.
3. Using your transfer pipette place seven drops of citric acid in five wells.
4. Add one drop of phenolphthalein in each of those five wells.
5. Counting each drop, begin adding 0.2 M NaOH stirring the solution as you go.
6. When the solution stays pink despite stirring, the solution has been neutralized.
Record the number of drops that were added.
7. Use this data to calculate the experimental value of the gram molecular weight of
the citric acid.
Dominic Held
Calculations:
We diluted solutions to lower Molarity in previous labs. Give your calculations
for diluting a 1.0 M NaOH solution into a 0.2 M NaOH solution.
Trial
#1
#2
#3
#4
Average
GMW
# drops
C6H8O7
# drops
indicator
# drops
0.2M NaOH
moles acid
Molarity of
citric acid
Citric acid is a 100 g/L solution. NaOH has a normality of 1.
[Citric acid] = [NaOH] x (drops of NaOH/drops of citric acid)
GMW = Concentration (in g/L) of citric acid in solution x 3 equivalents
Normality of citric acid
Molarity of citric acid = 1 / (GMW x 1L/100g)
Normality of citric acid = ([NaOH] x drops NaOH)/ ([citric acid] x drops citric acid)
Follow-up:
1. What is the molecular weight of citric acid?
2. What is the molecular weight of NaOH?
3. How many OH- ions can one molecule of NaOH produce?
4. How many H+ ions can citric acid produce?
5. How does this affect the titration of citric acid with NaOH?
6. What was your experimental normality of the citric acid?
Dominic Held
Assessment:
Participation- Student is on task, working well with group. Instructor should not have to
settle disputes between group members on how and who will complete the objectives of
the exercise.
Lab Report- This includes data and calculations.
Essay questions: Each student will submit their responses to the essay questions.
Responses will be graded on completeness, thoughtfulness, grammar, punctuation, and
spelling.
Lab Report and essay questions will be submitted on time. No late work will be
accepted without a written excuse from a parent or doctor.
Participation in Data Collection ----------------25 pts
Lab Report ----------------------------------------50 pts
Essay Questions-----------------------------------25 pts
Total---------------------------------------------100 pts
Dominic Held
Sample Data
Trial
1
2
3
4
Average
GMW
C6H8O7
7
7
7
7
7
192.3
Indicator
1
1
1
1
1
Molarity of
citric acid
(0.2 x 54.5) / (0.521 x 7) = 2.99
0.2M NaOH
56
55
53
54
54.5
0.521
moles acid
1.60
1.51
1.51
1.54
1.56