Titration Gizmos Lab
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Jan 9, 2024
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Student Exploration: Titration
Vocabulary:
acid, analyte, base, dissociate, equivalence point, indicator, litmus paper, molarity,
neutralize, pH, strong acid, strong base, titrant, titration, titration curve, weak acid, weak base
Prior Knowledge Questions
(Do these BEFORE using the Gizmo.)
There are several definitions of acids and bases. According to the
Brønsted-Lowry definition, an
acid
is a substance that is capable of
donating a proton to another substance. A
base
is a substance that
accepts protons. When an acid and a base are combined, the acid is
neutralized
as the base accepts the protons produced by the acid.
One way to determine if a solution is acidic or basic is to use
litmus paper
, as shown above.
There are two types of litmus papers: red and blue.
How does litmus paper indicate an acid? Both of the strips turn red.
How does litmus paper indicate a neutral substance? The red strip remains red and the blue
strip remains blue as well.
How does litmus paper indicate a base? Both of the strips turn blue.
Gizmo Warm-up
Litmus is an example of an
indicator
, a substance that changes
color depending on its
pH
(pH is a measure of the concentration of
protons, or H
+
ions). In the
Titration
Gizmo, you will use indicators to
show how acids are neutralized by bases, and vice versa.
To begin, check that
1.00 M NaOH
is selected for the
Burette
,
Mystery HBr
is selected for the
Flask
, and
Bromthymol blue
is
selected for the
Indicator
.
1.
Look at the flask. What is the color of the bromthymol blue indicator? Yellow
Th
2.
What does this tell you about the pH of the solution in the flask? That the pH is below 6.0
Solutions with a pH below 7.0 are acidic, while those with a pH above 7.0 are basic.
3.
Move the slider on the burette to the top to add about 25 mL of NaOH to the flask. What
happens, and what does this tell you about the pH of the flask? The solution turns blue,
meaning that the pH of the flask is above 7.6
Activity A:
Acids and bases
Get the Gizmo ready:
●
Click
Reset
. Select
1.0 M HNO
3
for the
Burette
and
Mystery NaOH
for the
Flask
.
●
Select
Phenolphthalein
for the
Indicator
.
●
You will need a scientific calculator for this activity.
Introduction:
When most acids dissolve in water, they
dissociate
into ions. For example, nitric
acid (HNO
3
) dissociates into H
+
and NO
3
–
ions.
Question: How do acids and bases interact in solution?
1.
Calculate: Concentration is measured by
molarity
(M), or moles per liter. Brackets are also
used to symbolize molarity. For example, if 0.6 moles of HNO
3
are dissolved in a liter of
water, you would say [HNO
3
] = 0.6 M.
A.
Because HNO
3
is a
strong acid
, it dissociates almost completely in water. That
means the concentration of H
+
is very nearly equal to that of HNO
3
.
What is [H
+
] if [HNO
3
] is 0.01 M? 0.01 M
B.
The pH of a solution is equal to the negative log of H
+
concentration: pH = –log[H
+
]
What is the pH of this solution? (Use the “log” button on your calculator.) 2
C. What is the pH of a 0.6 M HNO
3
solution? 0.22
2.
Describe: The equation for the reaction of nitric acid (HNO
3
) and sodium hydroxide (NaOH)
is shown on the bottom right of the Gizmo.
A.
What are the reactants in this reaction? HNO
3
and NaOH
B.
What are the products of this reaction? NaNO
3
and H
2
O
3.
Measure: A
titration
can be used to determine the concentration of an acid or base by
measuring the amount of a solution with a known concentration, called the
titrant
, which
reacts completely with a solution of unknown concentration, called the
analyte
. The point at
which this occurs is called the
equivalence point
.
Carefully add HNO
3
into the flask until the phenolphthalein begins to lose its color. Stop
adding HNO
3
when the color change is permanent.
A.
How much (HNO
3
) was required to cause the indicator to change color? 8.9 mL
B.
What can you say about the pH before and after the last drop of HNO
3
was added?
Before the last drop the pH was above 8.2, and after the last drop the pH was below 8.2
4.
Explore: Click
Reset
and change the indicator to
Bromthymol blue
. Add exactly 8.8 mL of
HNO
3
to the flask.
A.
What does the color of the indicator tell you about the current pH of the flask?
The colour is blue, which means that the pH is above 7.6
B.
Add one more drop of HNO
3
. What does the color tell you about the pH now?
The colour is now yellow, which indicates that the pH is below 6.0
C. If you combine the results of this question with the results from question 3B, what do
you know about the total pH change caused by adding the last 0.1 mL of HNO
3
?
We know that the last drop of HNO
3
caused the pH level to change from 8.2 to below 6.0
5.
Apply: Water has a pH of 7. If 0.1 mL (about one drop) of 1.0 M HNO
3
is added to 100 mL of
water, the result is a solution with a concentration of 0.001 M HNO
3
.
A.
What is the pH of 0.001 M HNO
3
? 3
B.
How much did one drop of HNO
3
cause the pH of water to change? From 7 to 3.
C. How does this relate to what you determined in question 4C?
This relates to 4C because it shows how much even a single drop of HNO
3
can lower a
solution’s pH level! (It's quite a lot)
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6.
Explain: A
titration curve
is a graph of pH vs. volume
of titrant. The graph at right shows a typical titration
curve for the titration of a strong acid by a
strong
base
. (A strong base is one that has relatively high
dissociation in water.)
A.
How would you describe the shape of the
titration curve? In the beginning as the titrant is
added, there is a very small slope, indicating
that there isn't much change in the pH level.
But as you get closer to the equivalence point,
the slope becomes very steep, and that means
that the pH level is increasing very quickly.
Then the curve stabilizes and is back to its
original slope.
B.
Why do you think the titration curve has the shape it has? I think it has this shape
because in the beginning when the base is added, it will react with the acid and there
will still be lots of acid leftover, so the pH will stay low. But once the acid is
neutralized and there is none left, the base will be leftover causing the pH to increase
rapidly, as basic solutions have a pH between 7 and 14.
Activity B:
Determining
concentration
Get the Gizmo ready:
●
Click
Reset
. Select
1.00 M NaOH
for the
Burette
and
Mystery H
2
SO
4
for the
Flask
.
●
Select
Bromthymol blue
for the
Indicator
.
Introduction:
Adding a drop of strong acid or base into a neutralized solution is similar to
adding a drop of strong acid or base to water—it causes an abrupt change in pH. By using an
appropriate indicator, a chemist can tell when a solution is neutralized by monitoring its color.
Question: How is titration used to determine an unknown concentration?
1.
Measure: Titrate the sulfuric acid analyte (H
2
SO
4
) with the sodium hydroxide titrant (NaOH).
How much 1.00 M NaOH is needed to neutralize the H
2
SO
4
solution? 21.6 mL
2.
Interpret: The balanced equation for the reaction of H
2
SO
4
and NaOH is given at bottom
right. Based on this equation, how many moles of NaOH react with 1 mole of H
2
SO
4
? 2 mol
3.
Manipulate: Recall that molarity is equal to the number of moles of a substance dissolved in
one liter of solution: molarity = moles ÷ volume.
A.
Write an equation for determining the number of moles of NaOH that are added to
the flask based on [NaOH] and volume of NaOH titrant (mL NaOH):
Moles NaOH = [NaOH] x NaOH mL / 1000 mL
B.
Write a similar expression for the number of moles of H
2
SO
4
in the flask based on
[H
2
SO
4
] and the volume of H
2
SO
4
(mL).
Moles H
2
SO
4
= [H
2
SO
4
] x H
2
SO
4
mL / 1000 mL
C. Because there are twice as many moles of NaOH as moles of H
2
SO
4
in this reaction,
you can say:
Moles NaOH = 2 · Moles H
2
SO
4
Substitute your expressions from 3A and 3B into this equation and solve for [H
2
SO
4
]:
[NaOH] x NaOH mL / 1000 = 2 x [H
2
SO
4
] x H
2
SO
4
mL / 1000
[NaOH] x NaOH mL = 2 x [H
2
SO
4
] x H
2
SO
4
mL
[H
2
SO
4
] = [NaOH] x NaOH mL
2 x H
2
SO
4
mL
D. Now calculate [H
2
SO
4
] based on the data from the Gizmo.
[H
2
SO
4
] = 0.108 M
4.
Calculate: Select the
Worksheet
tab. This tab helps you calculate the analyte concentration.
●
Fill in the first set of boxes (“moles H
2
SO
4
” and “moles NaOH”) based on the
coefficients in the balanced equation. (If there is no coefficient, the value is 1.)
●
Record the appropriate volumes in the “mL NaOH” and “mL H
2
SO
4
” boxes.
●
Record the concentration of the titrant in the M NaOH box.
Click
Calculate
. What is the concentration listed? 0.108 M
Click
Check
. Is this the correct concentration? Yes
If you get an error message, revise your work until you get a correct value. (You may have to
redo the titration if you do not have the correct volume of titrant.)
5.
Practice: Perform the following titrations and determine the concentrations of the following
solutions. In each experiment, list the volume of titrant needed to neutralize the analyte and
the indicator used. Use the
Worksheet
tab of the Gizmo to calculate each analyte
concentration. Include all units.
Titrant
Analyte
Indicator
Titrant volume
Analyte
concentration
0.70 M KOH
HBr
Phenolphthalein
30.0 mL
0.210 M
0.50 M HCl
Ca(OH)
2
Phenolphthalein
8.4 mL
0.021 M
0.80 M H
2
SO
4
NaOH
Phenolphthalein
5.6 mL
0.090 M
6.
Apply: Once you know the concentration of a strong acid or a strong base, you can estimate
its pH. Use pH = –log
10
[H
+
] to calculate the pH of each of the strong acid mystery solutions
(
Mystery HBr
and
Mystery H2SO4
) based on the concentrations you determined in
questions 4 and 5. Check your answers with the Gizmo. (Because dissociation is not always
complete, your answers may vary slightly from values in the Gizmo.)
[H
2
SO
4
] = 0.108 M
pH H
2
SO
4
= 0.967
[HBr] = 0.210 M
pH HBr = 0.678
7.
Apply: For a strong base, the concentration of hydroxide ions [OH
–
] is roughly estimated to
be the same as the concentration of the base. The pH of a strong base is found with the
equation pH = 14 + log
10
[OH
–
]. Based on their concentrations, find the pH of each of the
strong bases. Check your answers with the Gizmo.
[Ca(OH)
2
] = 0.021 M
pH Ca(OH)
2
= 12.32
[NaOH] = 0.090 M
pH NaOH = 12.95
Activity C:
Weak acids and
bases
Get the Gizmo ready:
●
Click
Reset
.
●
Select
1.00 M NaOH
for the
Burette
and
Mystery
CH
3
COOH
for the
Flask
.
Introduction:
Unlike strong acids and bases,
weak acids
and
weak bases
dissociate relatively
little in water. Some ions are formed, but the remaining molecules remain whole. As a result, the
pH of a weak acid or base is closer to neutral than the pH of a strong acid or base.
When weak acids or bases react with strong bases or acids, the resulting salts often act as
bases or acids themselves, causing the pH at the equivalence point to vary from 7.0. This can
impact your choice of indicator.
Question: What happens when weak acids and bases are titrated?
1.
Gather data: For each indicator given in the Gizmo, what is the pH range over which it
changes color?
Bromthymol blue: 6.0 - 7.6
Methyl orange: 3.1 - 4.4
Phenolphthalein: 8.2
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2.
Interpret: The salt produced by the reaction of acetic acid and sodium hydroxide,
CH
3
COONa, is a weak base. As a result, the pH of the equivalence point is slightly basic.
The titration curve for this reaction is shown below:
Volume NaOH
A.
Why is methyl orange not a good indicator to use for this titration? Methyl orange
would not be a good indicator because it’s colour changes at a pH of 4.4. When
looking at this curve, we can see that the pH is above 4.4 way before the
equivalence point is reached, so this indicator wouldn’t be a good choice.
B.
What would be a better indicator to use, and why? Phenolphthalein would be a better
choice because it changes colour at a pH much closer to the pH of the equivalence
point which is around 9. That’s why it would be a better indicator.
3.
Experiment: Perform two titrations, the first using methyl orange as an indicator and the
second using phenolphthalein as an indicator. Record the volume required to reach a color
change with each indicator:
Volume NaOH (methyl orange): 2.0 mL
Volume NaOH (phenolphthalein): 24.3 mL
A.
Why did you get such different results with each indicator? For a weak acid titrated
by a strong base, the pH rises above 5 long before the equivalence point is reached.
So because of this, very little titrant is needed for methyl orange to change colour.
The phenolphthalein takes much longer to change colour and more accurately
depicts when the equivalence point actually occurs.
B.
Which value would you use to calculate the acetic acid concentration, and why?
I would use phenolphthalein to calculate the acetic acid concentration because its
colour would change when the equivalence point is reached.
4.
Infer: The salt produced by the reaction of a weak base and a strong acid is acidic.
A.
Based on this fact, what can you say about the equivalence point of this reaction?
The equivalence point will have a pH below 7.0.
B.
Which indicator would you use for a titration of a weak base such as NH
3
? Explain.
I would use methyl orange because it changes colour at a pH below 7.0
5.
Calculate: Use the Gizmo to find the concentration of the
Mystery CH
3
COOH
and the
Mystery NH
3
. List the titrant and indicator you used for each titration.
Titrant
Analyte
Indicator
Titrant volume
Analyte
concentration
1.00 M HNO
3
CH
3
COOH
Bromthymol blue
14.7 mL
0.147 M
1.00 M NaOH
NH
3
Phenolphthalein
24.3 mL
0.243 M
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