NLC Characteristics of a Buffered Solution Lab Questions

Characteristics of a Buffered SolutionStudent Name
Date
1
Data
Activity 1
Data Table 1
0.1 M
Acetic
Acid
Water
Initial Color
Initial pH
Color after 1
drop HCl
pH after 1
drop HCl
Color after 1
drop NaOH
pH after 1
drop NaOH
Color after 10
drops NaOH
pH after 10
drops NaOH
© 2016 Carolina Biological Supply Company
0.1 M
Sodium
Acetate
pH 3.7
Buffer
pH 4.7
Buffer
pH 5.7
Buffer
2
Data Table 2
pH 4.7 Buffer
Initial Color
Initial pH
Color after 1
drop NaOH
pH after 1 drop
NaOH
Color after 2
drops NaOH
pH after 2
drops NaOH
Color after 3
drops NaOH
pH after 3
drops NaOH
© 2016 Carolina Biological Supply Company
First Dilution
Second
Dilution
Third Dilution
3
pH 4.7 Buffer
First Dilution
Second
Dilution
Third Dilution
Color after 4
drops NaOH
pH after 4
drops NaOH
Color after 5
drops NaOH
pH after 5
drops NaOH
Concentration
of sodium
acetate
1. The pKa of acetic acid is 4.7. What is the buffering range of this acid and its
conjugate base? Explain your answer.
2. Calculate the volume of 6 M acetic acid needed to prepare 100 mL of a
0.10 M acetic acid (CH3COOH) solution.
3. Calculate the mass of sodium acetate (CH3COONa) required to prepare 50
mL of a 0.10 M sodium acetate solution.
4. Calculate the volume of 1 M sodium hydroxide (NaOH) needed to prepare
10 mL of a 0.10 M sodium hydroxide solution.
5. Calculate the volume of 1 M hydrochloric acid (HCl) needed to prepare 10
mL of a 0.10 M hydrochloric acid solution.
© 2016 Carolina Biological Supply Company
4
6. Determine the volumes of 0.10 M CH3COOH and 0.10 M CH3COONa required
to prepare 10 mL of the following pH buffers. (Note: the pKa of CH3COOH
=4.7)
a. pH 3.7
b. pH 4.7
c. pH 5.7
7. Write a balanced equation for the dissociation equilibrium of acetic acid.
8. In Activity 2, there were 2 mL of buffer per well. Calculate the final pH of the
undiluted buffer after 0.25 mL of 0.1 M NaOH is added. The Ka of CH3COOH =
1.8 × 10-5. Use the CH3COOH buffer concentration calculated in Data Table
2, a balanced CH3COOH + NaOH reaction equation, an ICE table, and a
balanced dissociation equilibrium equation to help determine the pH.
9. What is the final pH of the second dilution after adding 1 drop of 0.1 M
NaOH?
© 2016 Carolina Biological Supply Company
5
Photos
Make sure the well plate is placed on a clean surface; for enhanced contrast, a
blank piece of paper can be placed under the plate.
Photo 1
Record the color of the solution for each row in the data table and take a
photo of the plate.
The following should be visible/indicated in this photo:
• Each row of the plate
• Each well in each of the rows
• The color differences in the wells
Photo 2
Record the color of the solution for each row in the data table and take a
photo of the plate.
The following should be visible/indicated in this photo:
• Each row of the plate
• Each well in each of the rows
• The color differences in the wells
Photo 3
Record the color of the first well of each row in the data table. You may need to
add an additional drop of indicator to each well to make the color more visible.
The following should be visible/indicated in this photo:
• Each row of the plate
• Each well in each of the rows
• The color differences in the wells
© 2016 Carolina Biological Supply Company
6
Photo 4
Record the color of the solution for each row in the data table and take a
photo of the plate.
The following should be visible/indicated in this photo:
• Each row of the plate
• Each well in each of the rows
• The color differences in the wells
© 2016 Carolina Biological Supply Company
Characteristics of a Buffered Solution
Student Name
Date
1
Data
Activity 1
Data Table 1
0.1 M
Acetic
Acid
Water
Initial Color
Initial pH
Color after 1
drop HCl
pH after 1
drop HCl
Color after 1
drop NaOH
pH after 1
drop NaOH
Color after 10
drops NaOH
pH after 10
drops NaOH
© 2016 Carolina Biological Supply Company
0.1 M
Sodium
Acetate
pH 3.7
Buffer
pH 4.7
Buffer
pH 5.7
Buffer
2
Data Table 2
pH 4.7 Buffer
Initial Color
Initial pH
Color after 1
drop NaOH
pH after 1 drop
NaOH
Color after 2
drops NaOH
pH after 2
drops NaOH
Color after 3
drops NaOH
pH after 3
drops NaOH
© 2016 Carolina Biological Supply Company
First Dilution
Second
Dilution
Third Dilution
3
pH 4.7 Buffer
First Dilution
Second
Dilution
Third Dilution
Color after 4
drops NaOH
pH after 4
drops NaOH
Color after 5
drops NaOH
pH after 5
drops NaOH
Concentration
of sodium
acetate
1. The pKa of acetic acid is 4.7. What is the buffering range of this acid and its
conjugate base? Explain your answer.
2. Calculate the volume of 6 M acetic acid needed to prepare 100 mL of a
0.10 M acetic acid (CH3COOH) solution.
3. Calculate the mass of sodium acetate (CH3COONa) required to prepare 50
mL of a 0.10 M sodium acetate solution.
4. Calculate the volume of 1 M sodium hydroxide (NaOH) needed to prepare
10 mL of a 0.10 M sodium hydroxide solution.
5. Calculate the volume of 1 M hydrochloric acid (HCl) needed to prepare 10
mL of a 0.10 M hydrochloric acid solution.
© 2016 Carolina Biological Supply Company
4
6. Determine the volumes of 0.10 M CH3COOH and 0.10 M CH3COONa required
to prepare 10 mL of the following pH buffers. (Note: the pKa of CH3COOH
=4.7)
a. pH 3.7
b. pH 4.7
c. pH 5.7
7. Write a balanced equation for the dissociation equilibrium of acetic acid.
8. In Activity 2, there were 2 mL of buffer per well. Calculate the final pH of the
undiluted buffer after 0.25 mL of 0.1 M NaOH is added. The Ka of CH3COOH =
1.8 × 10-5. Use the CH3COOH buffer concentration calculated in Data Table
2, a balanced CH3COOH + NaOH reaction equation, an ICE table, and a
balanced dissociation equilibrium equation to help determine the pH.
9. What is the final pH of the second dilution after adding 1 drop of 0.1 M
NaOH?
© 2016 Carolina Biological Supply Company
5
Photos
Make sure the well plate is placed on a clean surface; for enhanced contrast, a
blank piece of paper can be placed under the plate.
Photo 1
Record the color of the solution for each row in the data table and take a
photo of the plate.
The following should be visible/indicated in this photo:
• Each row of the plate
• Each well in each of the rows
• The color differences in the wells
Photo 2
Record the color of the solution for each row in the data table and take a
photo of the plate.
The following should be visible/indicated in this photo:
• Each row of the plate
• Each well in each of the rows
• The color differences in the wells
Photo 3
Record the color of the first well of each row in the data table. You may need to
add an additional drop of indicator to each well to make the color more visible.
The following should be visible/indicated in this photo:
• Each row of the plate
• Each well in each of the rows
• The color differences in the wells
© 2016 Carolina Biological Supply Company
6
Photo 4
Record the color of the solution for each row in the data table and take a
photo of the plate.
The following should be visible/indicated in this photo:
• Each row of the plate
• Each well in each of the rows
• The color differences in the wells
© 2016 Carolina Biological Supply Company
CHEMISTRY
Characteristics of a Buffered
Solution
Investigation
Manual
CHARACTERISTICS OF A BUFFERED SOLUTION
Table of Contents
2
Overview
2
Outcomes
2
Time Requirements
3
Background
5
Materials
6
Safety
6
Preparation
7
Activity 1
10 Disposal and Cleanup
Overview
Discover the characteristics of buffered solutions and the
important role they play in the environment and living systems.
Stock solutions that will be the buffers are prepared. Then, on the
basis of calculations, buffers of targeted pH and concentrations
are prepared. These prepared buffers are then tested to demonstrate how a buffer maintains pH as compared to non-buffered
solutions. Three buffered solutions of varying concentrations will
be tested to determine the impact of concentration on buffering
capacity.
Outcomes
• Prepare buffered solutions of a given volume and concentration.
• Determine the proportions of each chemical needed to produce
the targeted buffering capacity.
Time Requirements
Preparation …………………………………………………………… 20 minutes
Activity 1 …………………………………………………………….. 120 minutes
Key
Personal protective
equipment
(PPE)
goggles gloves apron
follow
link to
video
photograph stopwatch
results and
required
submit
warning corrosion flammable toxic environment health hazard
Made ADA compliant by
NetCentric Technologies using
the CommonLook® software
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Background
A buffer is a solution that resists change in pH
upon addition of acidic or basic components.
Thus, a solution can be buffered to prevent large
shifts in pH upon the addition of H+ or OH ¯ .
Typically, buffered solutions are composed of
a weak acid and its conjugate base or a weak
base and its conjugate acid.
Buffered solutions prevent large shifts in pH
when H+ ions or OH¯ ions are added. Typically,
buffered solutions are composed of a weak acid
and its conjugate base or a weak base and its
conjugate acid. Buffers can also be prepared
from a weak acid with a small amount of strong
base, a weak base with a small amount of strong
acid, or from an amphoteric substance. An
amphoteric substance is an oxide that can act
as both an acid and a base. A buffered system
exists in equilibrium. Any acid added to the
solution will be neutralized by the weak base
[A¯ in the equations] causing a shift in equilibrium
towards the weak acid. Any base added will
be neutralized by the weak acid [HA in the
equations], shifting the equilibrium left.
Weak Acid/Conjugate Base Equation:
HA →
← H+ + A¯
←
If Acid is added to the buffer system:
HA ← H+ + A¯
If Base is added to the buffer system:
HA → H+ + A¯
The buffers used in this lab activity will be made
from a weak acid, acetic acid, and its conjugate
base, sodium acetate. The descriptions and
calculations in the background section will
refer to buffers prepared as a weak acid with
conjugate base.
It is important to remember that acids and bases
will dissociate in aqueous solutions. The acid
dissociation constant (Ka) is the equilibrium
constant of the dissociation reaction of an acid
in aqueous solution, representing a dynamic
balance between the acid in its intact and
dissociated states. Most buffers perform best
at a pH near the pKa (-log10 Ka, a logarithmic
representation of the acid dissociation constant)
of the acid used for their preparation. When
designing a buffer, the chemicals used should
have pKas that are close to the target pH.
Most buffers perform best at a pH near the pKa
of the acid or base used. When designing a
buffer, the chemicals selected should have a pKa
close to the targeted pH.
The pH of a buffered solution can be calculated
using the Henderson-Hasselbalch equation.
Note: Brackets in an equation such as [Acid]
refer to the concentration.
[Conjugate Base]
pH = pKa + log (
)
[Weak Acid]
As described above, when protons or hydroxide
ions are added to the solution the equilibrium
shifts and the ratio of acid to conjugate base is
altered. The stoichiometry of the new system
must be calculated before any equilibrium
calculations can be done. This means that the
equilibrium calculations must be performed prior
to calculating the pH of the solution.
When a buffer is prepared from a weak acid
and its conjugate base, there will be some
protons present in the buffer solution due to the
ionization of the weak acid.
continued on next page
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CHARACTERISTICS OF A BUFFERED SOLUTION
Background continued
Table 1.
+
–
CH3COOH(aq) →
← H (aq) + CH3COO (aq)
CH3COOH
H+
CH3COO¯
Initial
0.033 M
0
0.056 M
Change
-x
+x
+x
(0.033 – x) M
x
(0.056 + x) M
Equilibrium
HA →
← A¯ + H+
←
Because the equilibrium constant for this
reaction is based on the weak acid Ka, added
OH¯ ions combine with the protons to produce
water. This causes the ionization equilibrium to
shift to the right and more of the weak acid must
dissociate to reestablish the equilibrium.
An ICE table (Initial, Change, Equilibrium) can
be helpful in determining the concentration of
species when equilibrium shifts occur. In the
example below there is an acetic acid/acetate
buffer with an initial acetic acid concentration of
0.033 M and an initial acetate concentration of
0.056 M. Some concentration of base is added
to the system, which results in a loss of x on the
acid side of the equilibrium and adding x to both
species on base side (shown in Table 1). Using
the pKa and the calculated concentrations you
can then determine pH.
However, the Henderson-Hasselbalch equation
is typically used to design a buffer of a targeted
pH. If the target pH and the pKa of the acid are
inserted into the equation, then you can calculate the ratio of the acid to its conjugate base.
The buffering capacity of a solution indicates
the amount of H+ or OH¯ ions the buffer is able
to absorb without a significant pH change. The
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buffering capacity of a solution is determined
by the magnitude of [HA] and [A¯]. Therefore,
increasing the quantity of each species will
increase the buffering capacity. However, the
species ratio must be kept similar to maintain
the desired pH.
To prepare a buffered solution of a specific pH
and buffer capacity:
1. Decide which conjugate acid-base pair to
use. A buffer is most effective when the
acid-base pair have approximately equal
concentrations. For this reason, it is best to
select a compound with a pKa that is within
one pH unit of the desired pH. The buffering
range is usually within ±1 pH unit of the pKa.
2. Next, decide the concentration of the buffer.
Since the buffer component concentration
determines its buffering capacity, the amount
of added H+ or OH¯ that the buffered solution
could encounter should be calculated.
Reorder Information: Replacement supplies
for the Characteristics of a Buffered Solution
activity can be ordered from Carolina
Biological Supply Company, kit 580324.
Call 800-334-5551 to order.
Materials
Needed from the equipment kit:
Included in the materials kit:
Acetic acid,
0.1 M, 120 mL
Sodium
acetate, 10 g
Indicator chart
24-well plate
Bogen’s
indicator
solution,
15 mL
Digital
balance
Graduated
Graduated
cylinder, 10 mL cylinder, 50 mL
Weigh boat
8 Pipets
3 Small plastic
cups, 1 oz
Wax pencil
8 Plastic cups,
10 oz
Needed from chemical set #2:
Sodium
hydroxide
solution, 1 M
Hydrochloric
acid solution,
1M
Needed but not supplied:
• Blank white paper
• Water, bottled or purified
There are many solids found in tap water that
will affect your calculations. Bottled, purified,
or filtered water from a home water purifier
(e.g., Brita® or PUR®) should be used.
www.carolina.com/distancelearning 5
CHARACTERISTICS OF A BUFFERED SOLUTION
Safety
Wear your safety
goggles, chemical
apron, and gloves at all times while conducting
this investigation.
Read all of the instructions for this laboratory
activity before beginning. Follow the instructions
closely, and observe all established laboratory
safety practices, including use of appropriate
personal protective equipment as described in
the Safety and Activity sections.
Hydrochloric acid and sodium
hydroxide are corrosive. In the event
of contact with skin or eyes, the
affected area should be immediately
flushed with water for 15 minutes.
Sodium acetate can be harmful if
swallowed and is a serious eye irritant.
Do not eat, drink, or chew gum while performing
this activity. Wash your hands with soap
and water before and after completing the
investigation, and clean up the work area with
soap and water. Keep pets and children away
from lab materials and equipment.
Preparation
1. Read through the Activity completely.
2. Collect all the materials.
3. Clean and sanitize the work area.
4. Label the eight plastic cups with the solution
names. The eight solutions are:
1. water
2. 0.1 M acetic acid
3. 0.1 M sodium acetate
• Calculate the mass of sodium acetate
(CH3COONa) required to prepare 50 mL
of a 0.10 M sodium acetate solution, the
molar mass is 82.03 g/mol.
4. 0.1 M NaOH
5. 0.1M HCl
6. pH 3.7 buffer
7. pH 4.7 buffer
8. pH 5.7 buffer
5. Determine the volumes of 0.10 M CH3COOH
and 0.10 M CH3COONa required to prepare
10 mL of the following pH buffers: pH 4.7, pH
5.7. (Note: the pKa of CH3COOH = 4.7.) Use the
Henderson-Hasselbalch equation to calculate
these volumes. An Example Calculation is given
below for the 3.7 pH buffer in Figure 1.
Figure 1.
6. The individual pipets should only be used
with a single solution. Label them to prevent
confusion and cross-contamination.
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ACTIVITY
ACTIVITY 1
A Buffer Solution Preparation
1. Based on the Preparation calculations,
prepare 50 mL of a 0.10 M solution of sodium
acetate in the cup labeled sodium acetate.
Mix with a pipet until completely dissolved.
a. Weigh out the calculated amount of
sodium acetate using a weigh boat and
the electronic balance.
b. Pour the sodium acetate into the cup and
add approximately 45 mL of pure water.
c. Mix the solution with a pipet until
completely dissolved.
d. Pour the contents of the cup back into
the cylinder and add water until the volume
reaches 50 mL and return the solution to the
cup.
e. Rinse the graduated cylinder with pure
water to avoid any cross contamination.
2. Add approximately 100 mL of pure water to
the cup labeled water.
3. Use the 50-mL graduated cylinder to transfer
50 mL of 0.1 M acetic acid to the cup labeled
acetic acid.
4. Add 10 drops of Bogen’s Universal Indicator
to the acetic acid and the sodium acetate
solutions. Add 20 drops of universal indicator
to the 100 mL of water in the cup.
5. Prepare 10 mL of 0.1 M NaOH and
0.1 M HCl.
a. Measure 1 mL of 1 M NaOH in the 10-mL
graduated cylinder.
b. Fill the graduated cylinder to 10 mL with
pure water.
c. Pour solution into the appropriately labeled
cups.
d. Rinse graduated cylinder thoroughly with
pure water.
e. Repeat dilution with 1 mL of 1 M HCl.
6. Add 2 drops of universal indicator to the
NaOH and the HCl solutions.
7. Prepare the 3.7, 4.7, and 5.7 pH buffer
solutions as calculated in the Preparation
section. Pour the solutions into their
respectively labeled cups.
The pipets are useful for transferring the
stock solutions to the graduated cylinder for
precise measurements. Each of the buffer
solutions should be a different color. Refer to
the indicator solution color chart to confirm
the pH of your prepared buffers.
8. Place the 24-well plate on to a sheet of
plain white paper. This will make it easier to
compare the solution colors. Orient the plate
vertically so that there are six wells per row
and four wells per column.
9. Add one drop of universal indicator to each
well. This ensures that the colors will be bright
when testing the solutions.
10. Use the graduation marks on the appropriate
pipets to transfer the solutions (see Figure 2).
a. 1 mL of 0.1 M acetic acid to all four wells
in the first row of the 24-well plate.
b. 1 mL of water to all four wells in the
second row of the plate.
c. 1 mL of 0.1 M sodium acetate solution
to all four wells in the third row.
d. 1 mL of the 3.7 pH buffer to all four wells
in the fourth row.
e. 1 mL of the 4.7 pH buffer to all four wells
in the fifth row.
continued on next page
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ACTIVITY
ACTIVITY 1 continued
Figure 2.
f. 1 mL of the 5.7 pH buffers to all four
wells in the sixth row.
Label the white paper with the solution
names in each column and row for easy
identification. Remember to only use the
pipet for its specific solution to avoid
cross-contamination.
1.
Record the color of the solution for
each row in Data Table 1 and take a
photo of the plate.
2. Match the row color to the indicator color
chart and determine the approximate pH of
the solution. Record this in Data Table 1.
3. Do not add anything else to the first well of
each row. These will be your color reference
for the rest of the rows.
8
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4. In the second well of each row, use the pipet
to add one (1) drop of 0.1 M HCl.
5. Keeping the base of the plate on the table,
gently swirl the solutions to mix them.
6. Record the resulting color and approximate
pH of each solution in Data Table 1.
7. In the third well of each row, add one (1) drop
of 0.1 M NaOH.
8. Gently swirl the solution and record the color
and approximate pH of each solution in your
Data Table 1.
9. Repeat the procedure in the fourth well of
each row adding ten (10) drops of 0.1 M
NaOH. Record the color and corresponding
pH on Data Table 1.
continued on next page
10.
Take a photo of the plate showing the
colors in each well.
11. Empty the plate into the sink with running
water. Rinse the plate with pure water, not
tap water.
12. Dry the plate by placing it upside down on
some paper towels. Tap it several times to
remove most of the water.
13. Set aside the water, acetic acid, sodium
acetate, and sodium hydroxide solutions.
You will need them for the next part of the
activity.
14. Empty the remaining plastic cups into the
sink with running water.
A Buffering Capacity of a Solution
1. Turn the well plate 90° so that it is four rows
by six columns.
2. In the cup labeled “4.7,” prepare 20 mL of
4.7 pH buffer by doubling the volumes
calculated in the prelab.
3. Add 2 mL of this solution into all 6 wells in the
first row. See Figure 3 below.
4. Dilute 5 mL of the 4.7 pH buffer solution
with 15 mL of purified water containing the
indicator and pour into a clean cup. This is
the first dilution.
5. Add 2 mL of the first dilution to all the wells in
the second row.
Figure 3.
continued on next page
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ACTIVITY
ACTIVITY 1 continued
6. Dilute 5 mL of the first dilution with 15 mL of
purified water containing indicator and pour
into a clean cup. This is the second dilution.
7. Add 2 mL of the second dilution to all the
wells in the third row.
8. Dilute 5 mL of the second dilution with 15 mL
of purified water containing indicator and pour
into a clean cup. This is the third dilution.
9. Add 2 mL of the third dilution to all the wells
in the fourth row.
10.
Record the color of the first well of
each row in Data Table 2. You may
need to add an additional drop of indicator
to each well to make the color more visible.
11. Add one (1) drop of 0.1 M NaOH solution to
the second well in each row.
Do not add any NaOH solution to the first well
of each row. This column will serve as a color
reference for the remaining wells in the row.
12. Add two (2) drops of 0.1 M NaOH solution to
the third well of each row.
13. Repeat with three (3) drops in the fourth well,
four (4) drops in the fifth well, and five (5)
drops in the sixth well.
14. Gently swirl the solutions until the color is
uniform.
15. Record the color and corresponding pH of
each well in the Data Table 2.
16.
Take a photo of the plate showing the
colors in each well.
10 Carolina Distance Learning
17. Calculate the sodium acetate molar
concentration of each dilution. Record
the calculated values in Data Table 2. The
concentration of the 4.7 pH buffer is 0.05 M
sodium acetate.
Concentration(start) x Volume(start) =
Concentration(final) x Volume(final)
Disposal and Cleanup
1. Dispose of solutions down the drain with the
water running. Allow the faucet to run a few
minutes to dilute the solutions.
2. Rinse and dry the lab equipment and return
the materials to your equipment kit.
Data Tables
Data Table 1: Preparation of Buffer Solutions
0.1 M
Acetic
Acid
Water
0.1 M
Sodium
Acetate
pH 3.7
Buffer
pH 4.7
Buffer
pH 5.7
Buffer
Initial
Color
Initial
pH
Color after
1 drop
HCl
pH after
1 drop
HCl
Color after
1 drop
NaOH
pH after
1 drop
NaOH
Color after
10 drops
NaOH
pH after
10 drops
NaOH
continued on next page
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Data Table 2: Buffering Capacity of a Solution
pH 4.7
Buffer
Initial
Color
Initial
pH
Color after
1 drop NaOH
pH after
1 drop NaOH
Color after
2 drops NaOH
pH after
2 drops NaOH
Color after
3 drops NaOH
pH after
3 drops NaOH
Color after
4 drops NaOH
pH after
4 drops NaOH
Color after
5 drops NaOH
pH after
5 drops NaOH
Concentration of
sodium acetate
12 Carolina Distance Learning
First
Dilution
Second
Dilution
Third
Dilution
NOTES
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13
NOTES
14 Carolina Distance Learning
NOTES
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15
CHEMISTRY
Characteristics of a Buffered Solution
Investigation Manual
www.carolina.com/distancelearning
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