Lab 3
INTRODUCTION TO ACIDS
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Gloves must be worn.
Students work in pairs.
OBJECTIVES
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Calculate the theoretical pH of acid and salt solutions
Use a pH meter and pH paper to measure the pH of a solution
Investigate how the pH and % ionization of an acidic solution change with dilution
Investigate the effect on pH when different amounts of a strong base are added to
polyprotic acid solutions
Learn how to carry out experiments to support or refute one’s predictions
Use evidence (i.e. data) to rationalize observations
INTRODUCTION
Acid-base chemistry plays an important role in many fundamental chemical processes, as
well as biochemistry, molecular biology, and cell biology. Many foods, household products, and
drugs are acids or bases, and the ability to donate or accept a proton is critical to the behavior of
these species.
Acids have long been recognized as an important class of chemicals. In fact the term acid
was first used in the 17th century. Historically acids have been categorized based on a series of
properties such as: sour taste, ability to turn blue litmus paper red, ability to dissolve many metals,
and ability to neutralize bases.
The first modern definition of an acid was formulated in the 1880s by Svante Arrhenius,
who proposed that an acid was “a substance that dissociates in water to form hydrogen ions (H+)”.
A second, more widely applicable definition is the Brønsted-Lowry definition, formulated in 1932,
which focuses on the transfer of H+, such that an acid is defined as a species that donates a proton
to another species, and the species that accepts the proton is defined as a base. The BrønstedLowry definition is not restricted to water, so it is more widely applicable than the Arrhenius
model, and it introduces the concept of a conjugate acid-base pair. Finally, the Lewis definition
(first articulated in 1923, but extended in 1938) broadens the definition of acids to species that
don’t contain hydrogen in their formula, such as BF3 and many metal coordination compounds.
The Lewis model states that an acid is an electron acceptor, and a base is an electron donor.
Many of the acids we will deal with in this course and in particular this section of the course
are Brønsted-Lowry acids and hence contain hydrogen in their formula. There are three important
points to understand about hydrogens in acids:
1) Although all Arrhenius acids contain hydrogen, not all hydrogen atoms in a substance are
capable of dissociating. One of the most important things for you to be able to do is identify
which proton(s) are capable of dissociating and predicting the relative ease with which this
Lab 3: Introduction to Acids
will happen.
2) As alluded to above, those hydrogens that do dissociate can do so to different degrees.
Strong acids are essentially 100% dissociated in solution. The degree of dissociation is
correlated with the strength of the acid. As a general rule of thumb, the more favorable it
is for a proton to dissociate, the stronger the acid.
3) Acids that possess more than one dissociable hydrogen atom are called polyprotic acids.
Sulfuric acid (H2SO4) and phosphoric acid (H3PO4) are common examples of polyprotic
acids. Intermediate forms such as HPO42- are called ampholytes (“amphi” from Greek
meaning “both sides”) and they are capable of both accepting and donating protons (i.e.
capable of acting as a base and an acid).
In this lab you will explore properties of acids, become familiar with measuring pH, explore
relative pH of different acids, and investigate the behavior of acids upon different perturbations
such as dilution and reaction with base. You will also experiment with the effect on pH when
different salts are added to water.
PROCEDURE
Part 1. Using the pH Meter: Calibration and Measurements
The first task in the investigation of acids and bases is to learn how to operate a pH meter. At
this time in preparation for the lab exercise, thoroughly read and become familiar with the
appendix on the pH meter. At the start of the lab period, the TA will check out a pH probe and
other special glassware. Please note that the probe is extremely fragile and expensive, so
handle it with care! The pH meter can take a long time to stabilize when going from a basic
solution to an acidic solution. In this lab, pH paper will also be used to measure pH. The pH
paper will confirm that the pH meter has stabilized. It is also used to determine the precision of
the two different methods for measuring pH.
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Calibrate the pH meter. Keep the calibration buffers until the end of the lab.
Place ~20 mL of each of the species listed below in separate clean, dry 18 x 150 mm test
tubes or 50 mL beakers.
1.
2.
3.
4.
5.
6.
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Tap water
0.5 M hydrochloric acid (HCl)
0.5 M formic acid (HCOOH)
0.5 M acetic acid (CH3COOH)
0.5 M sulfuric acid (H2SO4)
0.5 M phosphoric acid (H3PO4)
Measure the pH with the pH meter and pH paper.
Record the pH measurements in a data table in your laboratory notebook.
Lab 3: Introduction to Acids
Answer the following questions in your laboratory notebook:
1.1. Were the measured pH values consistent with the substance type?
1.2. Was there a significant difference between the pH measured by the pH meter and
the pH paper?
1.3. What are the potential sources of error measuring the pH with the pH meter and with
pH paper?
1.4. Under what circumstances might the pH paper or meter be a better choice for pH
measurement?
Part 2. Effect of Dilution
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Mix the following solutions in a 50 mL beaker, a large test tube or a 25 mL graduated
cylinder (test that the probe will fit before making the solution. Medium test tubes will
overflow!)
Dilute 2 mL of each acid listed below in 18 mL of DI water (10x dilution).
1. 0.5 M HCl
2. 0.5 M acetic acid
3. 0.5 M phosphoric acid.
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Measure the pH using the pH meter and record the data in a data table in the laboratory
notebook.
o Save these solutions for use in Part 3!
Dilute 1 mL of each acid listed above in 19 mL of DI water (20x dilution).
Mix the solutions in a 50 mL beaker, a large test tube or a 25 mL graduated cylinder.
Measure the pH using the pH meter and record the data in a data table in the laboratory
notebook.
Answer the following questions in the laboratory notebook:
2.1. Did the measured pH change agree with what you would have expected would happen?
2.2. After dilution, did the pH change more for the strong or weak acids? Suggest an
explanation for the observation.
Lab 3: Introduction to Acids
Part 3. Effect of Adding Base
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Add 1 mL of 0.5 M NaOH to the 10x diluted hydrochloric and phosphoric acid solutions
created in Part 2 of this experiment. Mix well.
Measure the pH of the solution, after the addition of NaOH, with the pH meter and record
the value in a data table in the laboratory notebook.
Add additional 1 mL increments of 0.5 M NaOH and record the pH after each addition,
until the pH of each solution is between 9.5 and 11.
Record the pH of the solution after each NaOH addition in a data table in the laboratory
notebook. (Note: this exercise mimics the titration experiment done in CHEM 1113 and
one done later in this course).
Address the following questions in the laboratory notebook:
3.1
3.2
What did you anticipate would happen to the pH as you added base?
Do hydrochloric acid and phosphoric acid require a similar amount of NaOH to reach
a pH between 9.5 and 11? Suggest an explanation for your observation.
Part 4. pH of Salt Solutions
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Obtain four clean 150 mm test tubes and fill them approximately halfway with:
1.
2.
3.
4.
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Tap water
Tap water + ~1 gram potassium chloride (KCl)
Tap water + ~1 gram sodium acetate (NaCH3COO)
Tap water + ~1 gram ammonium chloride (NH4Cl)
Mix each solution thoroughly making sure solids are dissolved.
Measure the pH of each solution with the pH meter and record in a data table in the
laboratory notebook (thoroughly rinse the electrode and pat dry between measurements).
Answer the following questions in your laboratory notebook:
4.1 What did you expect the pH of the solutions to be?
4.2 Which salts changed the pH of the DI water, and how did they change it? Write out
chemical formulas to explain the pH change.
Clean-up and Return Materials:
When you are finished collecting data, return the pH probe to its storage solution and disconnect it
from the LabQuest hand held system. Turn off the LabQuest device by pressing and holding the
power button located at the upper left of the screen. Return the LabQuest system, pH probe, and
any special glassware you used during this lab to the TA’s desk.
Lab 3: Introduction to Acids
STUDENT ASSIGNMENTS
I. Pre-laboratory Preparation
This assignment is due at the beginning of the laboratory period.
A. Pre-laboratory Assignment: Complete the pre-lab assignment in Canvas.
The following study questions are not graded but will help you prepare for the pre-lab assignment.
1.
Ka pKa
Reaction 1
HCl
+
H2O
Reaction 2
HCOOH
+
H2O
Reaction 3
CH3COOH
+
H2O
→
⇌  
⇌  
Reaction 4
H2SO4
+
H2O
→
H2O
⇌ 
?
?

Predicted
pH

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

















?
Reaction 5
H3PO4
+
Reaction 6
HCl
+
NaOH
→
Reaction 7
H3PO4
+
NaOH
→
Reaction 8
K+
+
H2O
→
Reaction 9
Cl-
+
H2O
→
Reaction 10
Na+
+
H2O
→
Reaction 11
CH3COO-
+
H2O
Reaction 12
NH4+
+
H2O
⇌ 
⇌ 
a. The aqueous reactions listed in the table above are studied in this laboratory experience.
Write the out the full reactions, including products, in your laboratory notebook. If no reaction
occurs, write NR after the arrow. (Note: the double and single headed arrows are important
and can help you understand what’s happening).
b. H2SO4 and H3PO4 are polyprotic acids. For those acids, add the additional dissociation
reactions that are missing to the table.
c. For the substances in the table, predict the pH in water. Do this without doing any
calculations! You will not be graded on whether or not your prediction matches the actual pH.
Your predictions are not right or wrong, they are simply based on your knowledge and
experience up to this point. Note: do not make predictions for reactions with NaOH.
Lab 3: Introduction to Acids
d. For those reactions in the table that are acid dissociations, look up the Ka values and record
them. Calculate the pKa values for those acids.
2. Calculate the theoretical pH of a 0.5 M solution of each acid in the table from Question 1.
3. Predict what will happen to the pH if each acid in Question 2 is diluted by 10x (i.e., a 0.05 M
solution of each acid). Will the solution be more acidic, less acidic, or stay the same? Predict
whether the % dissociation will increase, decrease, or stay the same. No calculations are
necessary! Only your prediction.
4. Will a strong acid always have a lower pH than a weak acid? Provide a rationale for the answer.
No calculations are necessary.
5. What happens to the pH of an acidic solution when a base such as NaOH is added? No
calculations are necessary.
6. Predict what will happen to the pH of Tap water when the salts listed below are dissolved. No
calculations are necessary.
a) KCl
b) NaCH3COO
c) NH4Cl
B. Prepare Data Section for lab
Prior to the start of lab, prepare the data collection section of the notebook. Format this section
for the collection of data during the lab, using the procedure section of the lab write-up and the
data collection section below as a guide for the data fields.
Lab 3: Introduction to Acids
II. Data Collection
Part 1: Using the pH Meter: Calibration and Measurements
Create a table (Data Table 1) to collect the following data for each solution in Part 1:
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Name of species
Volume and Molarity (mL, M)
pH measured by pH meter
pH estimated using pH paper
Part 2: Effect of Dilution
Create a table (Data Table 2) to collect the following data for each solution in Part 2:
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Name of species
How you prepared the 10x dilution (mL of 0.5 M solution, mL of H2O)
How you prepared the 20x dilution
pH of each solution as measured by the pH meter
Part 3: Effect of Adding a Base
Create a table (Data Table 3) to collect the following data for each solution in Part 3:
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Name of species
Volume and Molarity of starting solution
Volume of NaOH added to reach pH between 9.5-11
Moles of NaOH added for each of the above situations
pH measured by pH meter after each addition of 1 mL of NaOH
Part 4: pH of Salt Solutions
 Name of species
 pH of each solution as measured by the pH meter
III. Calculations
There are no additional calculations for this laboratory.
Lab 3: Introduction to Acids
IV.
Post-Laboratory Assignment
Prepare a section in your laboratory notebook to answer the following questions. Be sure to
explain your reasoning and write legibly. You do not have to copy the question into your
notebook, but be clear about which question you are answering.
V.
1.
Will a strong acid always have a lower pH than a weak acid? Use your data, results, and
calculations to provide evidence-based support for your answer.
2.
Phosphoric acid has three dissociation constants: Ka1, Ka2 and Ka3. What reactions do
these constants refer to? Why is one of them so much larger than the others? Which one
should you use to calculate the theoretical pH of a 0.50 M solution of phosphoric acid,
and why?
3.
The acid dissociation constant of chloroacetic acid, CH2ClCOOH, is Ka = 1.4 x 10-3.
Provide an explanation for the difference in Ka between chloroacetic acid and acetic acid.
(Hint: consider the structure of each molecule and the electronegativity of the atoms in
it).
Results and Conclusion
Write a one paragraph (3-5 sentences) statement that provides the scientific objective of the lab,
a brief summary of the procedure, and report the major results.
Lab 3: Introduction to Acids