Name: Date:

Student Exploration: Titration

Directions: Follow the instructions to go through the simulation. Respond to the questions and
prompts in the orange boxes.

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?

How does litmus paper indicate a neutral substance?

How does litmus paper indicate a base?

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?

2. What does this tell you about the pH of the solution in the flask?

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?

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 Get the Gizmo ready:
Activity A: ● Click Reset. Select 1.0 M HNO3 for the Burette
and Mystery NaOH for the Flask.
Acids and bases ● 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 (HNO3)
dissociates into H+ and NO3– 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 HNO3 are dissolved in a liter of water, you would say
[HNO3] = 0.6 M.

A. Because HNO3 is a strong acid, it dissociates almost completely in water. That means the
concentration of H+ is very nearly equal to that of HNO3.

What is [H+] if [HNO3] is 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.)

C. What is the pH of a 0.6 M HNO3 solution?

2. Describe: The equation for the reaction of nitric acid (HNO3) and sodium hydroxide (NaOH) is shown on the
bottom right of the Gizmo.

A. What are the reactants in this reaction?

B. What are the products of this reaction?

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 HNO3 into the flask until the phenolphthalein begins to lose its color. Stop adding HNO3 when
the color change is permanent.

A. How much (HNO3) was required to cause the

indicator to change color?

B. What can you say about the pH before and

after the last drop of HNO3 was added?

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4. Explore: Click Reset and change the indicator to Bromthymol blue. Add exactly 8.8 mL of HNO3 to the
flask.

A. What does the color of the indicator tell you about the current pH of the flask?

B. Add one more drop of HNO3. What does the color tell you about the pH now?

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 HNO3?

5. Apply: Water has a pH of 7. If 0.1 mL (about one drop) of 1.0 M HNO3 is added to 100 mL of water, the
result is a solution with a concentration of 0.001 M HNO3.

A. What is the pH of 0.001 M HNO3?

B. How much did one drop of HNO3 cause the pH of water to

change?

C. How does this relate to what you determined in question 4C?

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?

B. Why do you think the titration curve has the shape it has?

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 Activity B: Get the Gizmo ready:
● Click Reset. Select 1.00 M NaOH for the Burette
Determining and Mystery H2SO4 for the Flask.
concentration ● 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 (H2SO4) with the sodium hydroxide titrant (NaOH).

How much 1.00 M NaOH is needed to neutralize the H2SO4 solution?

2. Interpret: The balanced equation for the reaction of H2SO4 and NaOH is given at bottom right.

Based on this equation, how many moles of NaOH react with 1 mole of H2SO4?

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 =

B. Write a similar expression for the number of moles of H2SO4 in the flask based on [H2SO4] and the
volume of H2SO4 (mL).

Moles H2SO4 =

C. Because there are twice as many moles of NaOH as moles of H2SO4 in this reaction, you can say:

Moles NaOH = 2 · Moles H2SO4

Substitute your expressions from 3A and 3B into this equation and solve for [H2SO4]:

D. Now calculate [H2SO4] based on the data from the Gizmo.

[H2SO4] =

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4. Calculate: Select the Worksheet tab. This tab helps you calculate the analyte concentration.
● Fill in the first set of boxes (“moles H2SO4” 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 H2SO4” boxes.
● Record the concentration of the titrant in the M NaOH box.

Click Calculate. What is the concentration listed?

Click Check. Is this the correct concentration?

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.

Analyte
Titrant Analyte Indicator Titrant volume
concentration
0.70 M KOH HBr
0.50 M HCl Ca(OH)2
0.80 M H2SO4 NaOH

6. Apply: Once you know the concentration of a strong acid or a strong base, you can estimate its pH. Use pH
= –log10[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.)

[H2SO4] = pH H2SO4 = [HBr] = pH HBr =

7. Apply: For a strong base, the concentration of hydroxide ions [OH–] is estimated to be the same as the
concentration of the base. The pH of a strong base is found with the equation pH = 14 + log10[OH–]. Based
on their concentrations, find the [OH–] and pH of each of the strong bases. Check your answers with the
Gizmo.

[NaOH] = [OH-] = pH NaOH =

[Ca(OH)2] = [OH-]* = pH Ca(OH)2 =

*Note: For Ca(OH)2, the OH– concentration is double the Ca(OH)2 concentration because there are two
OH– ions in each Ca(OH)2 molecule.

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 Activity C: Get the Gizmo ready:
● Click Reset.
Weak acids and ● Select 1.00 M NaOH for the Burette and Mystery
bases CH3COOH 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: Methyl orange: Phenolphthalein:

2. Interpret: The salt produced by the reaction of acetic acid and sodium hydroxide is a weak base,
CH3COONa. 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?

B. What would be a better indicator to use, and why?

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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): Volume NaOH (phenolphthalein):

A. Why did you get such different results with each indicator?

B. Which value would you use to calculate the acetic acid concentration, and why?

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?

B. Which indicator would you use for a titration of a weak base such as NH3? Explain.

5. Calculate: Use the Gizmo to find the concentration of the Mystery CH3COOH and the Mystery NH3. List
the titrant and indicator you used for each titration.

Titrant Analyte Indicator Titrant volume Analyte concentration

CH3COOH

NH3

6. On your own: If you like, you can continue to practice titration calculations by selecting Random for the
Flask. Click New to change the analyte. Record your results in the space provided.

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