Identification of an Unknown Solid Acid
You will have two lab periods to complete this assignment. Work in groups of 2–3.
Introduction. In this experiment, you will design and perform acid-base titration experiments to identify
an unknown solid acid. Your group will be randomly assigned one of the acids listed in Table 1. Note that
all are weak, monoprotic acids.
Table 1: List of possible unknown solid acids. (MM = molar mass.)
Name of Acid
Formula
MM (g·mol–1) pKa
Benzoic acid
C7H6O2
122.12
4.204
Chloroacetic acid
C2H3ClO2 94.50
2.867
p-Chlorobenzoic acid
C7H5ClO2 156.57
3.986
Diphenylacetic acid
C14H12O2
212.25
3.939
2-Furoic acid
C5H4O3
112.08
3.164
Glycolic acid
C2H4O3
76.05
3.831
Hydrocinnamic acid
C9H10O2
150.18
4.664
Iodoacetic acid
C2H3IO2
185.95
3.175
l-Lactic acid
C3H6O3
90.08
3.858
dl-Mandelic acid
C8H8O3
152.15
3.370
Potassium bitartrate
C4H5KO6
188.18
4.360
Potassium hydrogen phthalate
C8H5KO4
204.23
5.408
Potassium phosphate monobasic H2KPO4
136.08
7.200
Suberic acid
C8H14O4
174.20
4.512
Sulfanilic acid
C6H7NO3S 173.19
3.227
2-methylbenzoic acid
C8H8O2
136.15
3.900
Trimethylacetic acid
C5H10O2
102.13
5.031
Objective. Identify an unknown solid acid using acid-base titration experiments.
THE RULES—read carefully!
1. Your group must perform both types of titrations:
1. Using an indicator dye, but no pH meter.
2. Using a pH meter, but no indicator dye.
You may not use both tools in the same titration.
2. You must validate your method on a known acid first.
You will be provided potassium hydrogen phthalate (KHP, MM = 204.23 g·mol–1, pKₐ = 5.408).
You must validate your experiments with KHP before you start working with your unknown.
3. Each titration must be performed at least 3 times.
In total, the minimum requirement is 12 titrations:
• 3 for the molar mass of KHP,
• 3 for the pKa of KHP,
• 3 for the molar mass of the unknown acid,
• 3 for the pKa of the unknown acid.
More trials are permitted—fewer are not.
4. Each group member must personally perform:
1. At least one experiment determining the molar mass of the unknown acid.
2. At least one experiment determining the pKa of the unknown acid.
Unknown Acids, Indicators, and Buffers
page 2
Design Considerations:
• Acid-base titrations can be used to determine the molar mass and the pKₐ of acidic compounds. The
textbook discusses these types of experiments at length.
• You need to know exactly how much acid you are titrating. Will you weigh a fresh sample each time?
Or prepare a stock solution? If making a stock solution, use volumetric glassware. Do not rely on
graduated cylinders for precise measurements!
• Your buret holds up to 50.00 mL of solution and is labeled in 0.1-mL increments. Make measurements
to the nearest 0.01 mL by eye. For accuracy, perform titrations requiring a total of ~10–20 mL of added
acid or base solution. A 0.9-mL titration is difficult to measure precisely by eye, whereas a 75-mL
titration is wasteful and less precise.
Suggestions:
• Aim to determine the molar mass and pKa of KHP during Week 1. If you cannot reproduce the accepted
values within 5–10% error, revise your procedure and try again.
• Coordinate with your lab group before you arrive in the lab.
• Calculate the amount of KHP needed for a ~10–20 mL titration before you arrive in the lab.
• Use the same approximate mass for your unknown (since you don’t know its molar mass yet).
• Do not attempt to weigh out exactly the calculated amount of acid. Instead, weigh out approximately
that amount and record the exact mass in your notebook.
• Ensure all the solid dissolves before titrating. Some solid acids are more soluble than others—gentle
heating may be necessary in some cases. Undissolved solids will lead to invalid data.
• Do not attempt to fill your buret exactly to the 0.00-mL mark. Instead, fill it to any point below the
0.00-mL mark, record the starting and the final volumes to the nearest 0.01 mL, and calculate the
volume added by difference.
• Your group will have only one pH meter—plan to start those experiments as soon as possible.
• Apply the same approach to your unknown. If your experiments do not provide consistent results, you
might need to reconsider your approach.
Available Equipment and Reagents:
You may use any equipment in your lab drawer in addition to the following items:
• Standardized acid solution: ~0.10 M HCl (Exact concentration listed on label.)
• Standardized base solution: ~0.10 M NaOH (Exact concentration listed on label.)
• Indicator solutions: Methyl orange, thymol blue, and phenolphthalein.
• Unknown solid acid. (Pure solid, assigned by your TA.)
• One pH meter per team
• One UV-Vis spectrophotometer per team
• 100.0 mL volumetric flasks
• 25.00 mL volumetric flasks
• 50.00 mL burets
Safety Considerations and Waste Disposal:
If the acids or bases contact your skin, rinse the affected areas under running water. Please wear gloves
when handling the solid acids, as some are caustic. All chemical waste generated during this experiment
should be flushed down the sink.
Unknown Acids, Indicators, and Buffers
page 3
ASSIGNMENTS
All lab reports must be written individually. Data that are recorded as a group should be attributed in the lab report to
all members of the group. Data that are recorded by a specific group member and shared with the group must be
explicitly attributed to the group member who did the work. All text, tables, and figures in your lab report should be
prepared by you; copying these materials from another student or sharing your materials with another student is a
violation of the Cornell Code of Academic Integrity.
1. PRELAB (15 pts)
Submit your work as a single PDF document to the “PRELAB” assignment in the E4 module on Canvas.
In your lab notebook, write up tentative procedures for two titration experiments designed to determine (1)
the molar mass and (2) the pKa of KHP. Your procedures must be detailed enough to enable any general
chemistry student to carry out the experiment exactly as written. Clearly describe how you will prepare the
analyte solution(s), including specific masses, volumes, and concentrations. Show all necessary
calculations and explicitly state which standardized titrant solution you will use. Submissions will be
evaluated on the soundness of the experimental design, clarity of writing, level of detail, legibility, and
organization.
Your notebook must be prepared according to the guidelines in the Reports, Graphs, Notebooks resource
on Canvas (see pages 15–17). Be sure to include all required content outlined under “Setting up your
Notebook: Weekly” on page 17.
For this PRELAB, you may either handwrite or type your procedures. If typed, they must be printed, cut
out neatly, and taped into your notebook. Regardless of the format, submit scans of your fully prepared
notebook pages only.
2. CHECKPOINT (15 pts)
Submit your work, typed, as a single PDF document to the “CHECKPOINT” assignment in the E4 module
on Canvas. Notebook scans are not required.
a. Prepare a typed, report-level draft of your experimental methods for determining the molar mass
and pKa of KHP. Your procedures should reflect any adjustments you made during Week 1.
b. Using your experimental data, demonstrate how you determined the molar mass and the pKa of
KHP. Include all necessary data, calculations, and any plots used in your analysis.
3. REPORT (70 pts)
After the second week of lab, you will be submitting a formal lab report comprised of the following:
a. Abstract.
b. Experimental methods section.
c. A unified “Results and Discussion” section. Be sure to identify potential sources of experimental
error and discuss specifically how they would affect your results. If possible, you should also
suggest improvements to your procedure.
d. Sample calculations (i.e., not every single calculation—if a calculation is performed multiple times,
say on replicate data, it is sufficient to show just one as an example).
Submit your work, typed, as a single PDF document to the “REPORT” assignment in the E4 module on
Canvas. More information and examples are provided in the Reports, Graphs, Notebooks resource on
Canvas. Notebook scans are not required.