Back to learning exercises hit list
Search all MERLOT
Click here to go to your profile
Select to go to your workspace
Click here to go to your Dashboard Report
Click here to go to your Content Builder
Click here to log out
Search Terms
Enter username
Enter password
Please give at least one keyword of at least three characters for the search to work with. The more keywords you give, the better the search will work for you.
select OK to launch help window
cancel help

MERLOT II


    

Learning Exercise


Material: Virtual Chemistry Laboratory
Submitted by: Moustapha Diack on Mar 04, 2004
Date Last Modified: Feb 24, 2011
Title: Chemistry of Acids and Bases
Description: In this Pre-lab you will design and conduct the following acid base titration:
1) Hydrochloric acid, HCl (strong acid), with sodium hydroxide, NaOH (strong base)
2) Hydrochloric acid, HCl (strong acid), with ammonia, NH3 (weak base)
3) Acetic acid, HC2H3O2 (weak acid), with sodium hydroxide, NaOH (strong base)
4) Acetic acid, HC2H3O2 (weak acid), with ammonia, NH3 (weak base)

Students design their own experiments to complete the titration experiments. They a formal laboratory report addressing the following questions.
In which trial(s) did Phenolphthalein change color at about the same time as the large increase in pH occurred at the equivalence point? In which trial(s) was there a significant difference in these two times? Suggest and test a more suitable indicator for the designed experiments. The stock room of the virtual lab features other indicators such as Bromocresol Green, Cresol Red, Methyl Orange and Methyl Red. By examining a titration curve, how can you decide which acid-base indicator to use to find the equivalence point?
Of the four titration curves, which combination of strong or weak acids and bases had the longest vertical region of the equivalence point? The shortest? The acid-base reaction between HCl and NaOH produces a solution with a pH of 7 at the equivalence point (NaCl + H2O). Why does an acid-base indicator that changes color at pH 5 or 9 work just as well for this reaction as one that changes color at pH 7? In general, how does the shape of a curve with a weak specie (NH3 or HC2H3O2) differ from the shape of a curve with a strong specie (NaOH or HCl)? Write each of the equations for the acid/base titrations. Use a data acquisition program to record Vol NaOH vs. pH and plot the titration curves.
Time Required 3 hours
Topics: Chemistry Pre-Laboratory Assignment
Course: Second Year General Chemistry
Prerequisites Skills: Foundation of Acid and Base Chemistry. User are referred to www.chemtutor.com for content reinforcement and support for this assignment.
Learning Objectives: This Pre-laboratory uses a virtual laboratory. In completing the assignment, the student will be able to:

1.Prepare standard solutions of acids and bases from stock solutions using dilution method.
2. Design acid/base titration experiments, collect experimental data, observe differences in shapes of titration curves when various strengths of acids and bases are combined and be able to interpret the shape of a titration curve.
3. Learn about the function and selection of appropriate acid-base indicators in titration experiments.
4. Apply acid base titration and pH concepts in quantitative problem solving.
Text of Learning Exercise: Pre- Laboratory Assignment
The Chemistry of Acids and Bases
Developed by M. Diack 1,2 Somuri Venkata Aswani 3
Depts. Of 1Chemistry and 2Science/Math Education- 3 Department of Computer Science
Southern University, Baton Rouge, LA 70813
>

A. Objectives
This Pre-laboratory uses a virtual laboratory. In completing the assignment, the student will be able to:

1.Prepare standard solutions of acids and bases from stock solutions using dilution method.
2.Design acid/base titration experiments, collect experimental data, observe differences in shapes of titration curves when various strengths of acids and bases are combined and be able to interpret the shape of a titration curve.
3.Learn about the function and selection of appropriate acid-base indicators in titration experiments.
4.Apply acid base titration and pH concepts in quantitative problem solving.


B. Virtual Experiments and Procedure

In this Pre-lab you will design and conduct the following acid base titration:
1)Hydrochloric acid, HCl (strong acid), with sodium hydroxide, NaOH (strong base)
2)Hydrochloric acid, HCl (strong acid), with ammonia, NH3 (weak base)
3)Acetic acid, HC2H3O2 (weak acid), with sodium hydroxide, NaOH (strong base)
4)Acetic acid, HC2H3O2 (weak acid), with ammonia, NH3 (weak base)


B1. Solutions Preparation

Using the virtual lab stock room, prepare 250 mL of 0.1 M solutions of hydrochloric acid, HCl - acetic acid, HC2H3O2, and ammonia, NH3, from stock solution of HCl (3 M), HC2H3O2 (3M), and NH3 (3 M) available in the Virtual Chemistry Laboratory Volumetric flasks of 250 mL are available for the preparation of the solutions. A standard solution 0.1 M sodium hydroxide, NaOH, available in the Virtual Lab Stock will be used for titration experiments.


B2. Titration Experiments


B2.1 Strong Base (titrant)/Strong Acid


A titration is a process used to determine the volume of a solution needed to react with a given amount of another substance. In this experiment, you will titrate hydrochloric acid solution, HCl, you prepared in B1, with standard solution of NaOH 0.1 M. The concentration of the NaOH solution is standard and you will determine the unknown concentration of the HCl using titration. Hydrogen ions from the HCl react with hydroxide ions from the NaOH in a one-to-one ratio to produce water in the overall reaction:

H+(aq) + Cl?(aq) + Na+(aq) +OH?(aq) H2O(l) + Na+(aq) + Cl?(aq)

When an HCl solution is titrated with an NaOH solution, the pH of the acidic solution is initially low. As base is added, the change in pH is quite gradual until close to the equivalence point, when equimolar amounts of acid and base have been mixed. Near the equivalence point, the pH increases very rapidly, as shown in Figure 1. The change in pH then becomes more gradual again, before leveling off with the addition of excess base. The region of most rapid pH change will then be used to determine the equivalence point. The volume of NaOH titrant used at the equivalence point will be used to determine the molarity of the HCl.


FIGURE 1
Shows a graphical representation of a titration curve (Strong Acid/Strong Base)



Using the Virtual Lab prepare your experimental setup as shown below in figure 2 and complete the experiment. (a) Place 10 mL of 0.1 M HCl solution into a 250-mL beaker and add about 100 mL of distilled water. (b) Add 3 drops of phenolphthalein acid-base indicator. (c) fill a 50-mL Buret with 0.1 M standard solution of NaOH. (d) complete your titration pass the equivalent point and collect pH data versus Volume of NaOH added. (e) Using the Logger Pro data collection and Analysis software, input your experimental data and plot the graph of the titration curves.

Processing the Data:

1. Use your graph and data table to determine the volume of NaOH titrant used in each trial. Examine the data to find the largest increase in pH values upon the addition of 1 drop of NaOH solution. Find and record the NaOH volume just before this jump.

2. Find and record the NaOH volume after the drop producing the largest pH increase was added.

3. Determine the volume of NaOH added at the equivalence point. To do this, add the two NaOH values determined above and divide by two.

4. Calculate the number of moles of NaOH used.

5.See the equation for the neutralization reaction given in the introduction. Determine the number of moles of HCl used.

6.Recall that you pipeted out 10.0 mL of the unknown HCl solution for each titration. Calculate the HCl concentration.

7.If you did two titrations, determine the average [HCl] in mol/L.


FIGURE 2: EXPERIMENTAL SETUP
Figure 2 shows a Snaphshot of the Virtual Laboratory Bench with a classical titration setup with all the parts labelled.


Note: Phenolphthalein is a suitable indicator for this reaction, it clearly changes from clear to red at about the same time as the jump in pH occurs (equivalent point). In your data table, record the exact pH and Volume NaOH at which the phenolphthalein indicator changes color. Watch to see and confirm if the phenolphthalein changes color before, at the same time, or after the rapid change in pH at the equivalence point. Phenolphthalein changes from clear to red at a pH value of about 9. Drawing a horizontal line from a pH value of 9 on the vertical axis to its intersection with the strong base/strong acid titration curve. In which trial the line intersect the nearly vertical region of the curve, clearly indicating that phenolphthalein is suitable.

B2.2 Weak Base(titrant)/Strong Acid
B2.3 Strong Base(titrant)/Weak Acid
B2.4 Weak Base(titrant)/Weak Acid


Design your own experiments to complete B2.2, B2.3 and B2.4 titration experiments. Write an experiment report addressing the following questions for the experiments.
In which trial(s) did Phenolphthalein change color at about the same time as the large increase in pH occurred at the equivalence point? In which trial(s) was there a significant difference in these two times? Suggest and test a more suitable indicator for experiments B2.2, B2.3, and B2.4. The stock room of the virtual lab features other indicators such as Bromocresol Green, Cresol Red, Methyl Orange and Methyl Red. By examining a titration curve, how can you decide which acid-base indicator to use to find the equivalence point?
Of the four titration curves, which combination of strong or weak acids and bases had the longest vertical region of the equivalence point? The shortest?
The acid-base reaction between HCl and NaOH produces a solution with a pH of 7 at the equivalence point (NaCl + H2O). Why does an acid-base indicator that changes color at pH 5 or 9 work just as well for this reaction as one that changes color at pH 7?
In general, how does the shape of a curve with a weak specie (NH3 or HC2H3O2) differ from the shape of a curve with a strong specie (NaOH or HCl)?
Write each of the equations for the acid/base titrations.


C. Virtual Laboratory and Web Support


C1. Download and Install the Virtual Lab.
http://ir.chem.cmu.edu/irproject/applets/virtuallab/Download.asp

An Instructor-led tutorial will be provided. Also an online tutorial is available at the target link above.


C2. Content resource on the Chemistry of Acid/Bases is available at the Internet Resource link: http://www.chemtutor.com/acid.htm#p


D. Laboratory Report

You are required to write a formal laboratory report for this Pre-laboratory assignment. Notice that all of the questions posed addressed in the report and are included simply as a logical part of the story. Reports should include the titration curve to be generated using the
http://www.cchem.berkeley.edu/chem1a/fall03/labsamplewrite.htm
Bookmark and Share