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Comparative Study of Common Household Solutions
A Volumetric Titration

Description of Activity

Title:

Comparative Study of Common Household Solutions--A Volumetric Titration.

Overview:

Working in small groups (2-4 students) students will design an experiment which measures and compares the amount of acid or base in two household products. Suggested comparisons may include (but are not limited to):

Strengths of two different brands of vinegar.
Ammonia concentration in two different household ammonia cleaners.
Acidity of two different cola drinks (brands or diet/sugared).
Effectiveness of two different antacid tablets.
Acidity in rain samples from two geographic areas.
Buffering in two different brands of aspirin.
Etc.

Students will chose a comparison, propose a hypothesis, design and carry out a titration to test their hypothesis and report those results.

The skill development lab experience together with the student planned activity should take about four 50 minute class periods. Time should be provided for group planning as well as sufficient lab time.

Materials:

Skill Development Activity:
0.100 M NaOH (40.0 grams NaOH dissolved to 1.000 L). This solution should be prepared the day of the lab to prevent absorbtion of carbon dioxide.
Approximately 0.1 M HCl (to be standardized by students)
Phenolphthalein
Distilled Water

Macro-Scale Materials		Micro-Scale Materials
Burets				24-well micro-plate
Buret Stand			96 well micro-plate
Double Buret Clamp		micro pipets (this stem)
Graduated Cylinder		toothpicks (plastic)
Erlenmeyer Flask		plastic drinking straws
Beakers				scissors
Dropper Pipet			hole punch
Suction Bulb

Inquiry Lab Activity:

All materials used in the verification experiments should be available for student use. Additionally students will need to bring from home the household substances that they choose to analyze and compare.

If additional solutions are prepared they will need to be standardized either by students or the instructor.

Background

Volumetric titration is a method of analyzing the amount of a substance (such as an acid or a reducer) based upon a reaction with a known concentration of a second substance ( such as a base or an oxidizer). Complete reaction occurs when the two reactants are in exact stoichiometric proportions in solution. The end point of the reaction is indicated by a color change, either of one of the reactants or of an additional indicator (e.g. phenolphthalein in a neutralization reaction). In an acid-base titration the end point is very close to were the number of moles of hydronium ions from the acid equals the number of hydroxide ions from the base. At this point the volume of the standard solution used is carefully determined. Then, the measured volumes of the two solutions and the known concentration of the standard solution can be used to calculate the concentration of the other solution. The following steps tell how to calculate the unknown concentration:

1. Write the balanced equation for the reaction. From the coefficients, determine how many moles of acid reacts with 1 mole of base (or vice versa). Use the coefficients to form a mole ratio.
2. If the mole ratio is 1:1, the following relationship can be used to calculate the unknown concentrations
Ma X Va = Mb X Vb
where Ma = the molarity of the acid
Mb = the molarity of the base
Va = the volume of the acid
Vb = the volume of the base

3. If the mole ratio is not 1:1, the calculation of the unknown molarity is slightly more complicated. But remember that the moles of H+ must equal the moles of OH-.

Teaching and Learning Strategies

If students intend to test colored foods such as tomato juice or orange juice, they will need to develop a strategy for resolving the color problem.

Prerequisite instruction:

Students should be competent in the laboratory techniques required to perform an acid base titration. They should also be able to mathematically interpret the data resulting form such a titration.

The instructor should be very careful in giving suggestions to students as they design their own experiment. Encourage imagination and creativity.

Household products which contain weak acids or weak bases are extremely common. Students should be encouraged to develop a sense of curiosity and imagination in developing a comparative plan. Keep in mind that "no significant difference in the products with the given test" is a valid conclusion.

Safe Operating Procedures

Goggles and aprons must be worn in the laboratory at all times. Gloves are also recommended.

The acids and bases used are harmful to your skin, eyes, and nose. Wash all spills immediately with generous amounts of tap water.

Acids and bases should be neutralized before putting into the central sewage system. Dispose of all materials properly. See Flinn Chemical Catalog for details.

Skills Development Lab

Instructions for making a micro-scale buret can be found in many micro-scale laboratory manuals.

Micro-scale Procedures

1. With drinking straws, construct a ringstand following the instructions found in the Micro scale manual. Set up the ringstand in well A1 in the 96-well micro-plate.
2. Fill a micropipet with 0.1 NaOH solution and mount the pipet in the straw holder.
3. Place a 24 well micro-plate on top of a white piece of paper. Place 10 drops of acid solution in each of wells A1, B1, C1, and D1.
4. Using a clean pipet add 1 drop only of phenolphthalein indicator to each of the wells with the acid. Mix the contents of the wells with a toothpick.
5. Move the 96 well micro-plate with the pipet containing 0.1 M NaOH solution to a position where the pipet is over well A1 containing the acid.
6. Add 0.1 M NaOH solution in the pipet to the acid drop by drop. Stir the well with the toothpick and count the number of drops as you titrate your sample to the end point.
7. Record the number of drops of NaOH used to titrate A1.
8. Repeat the process for B1, C1, and D1. Record your results for each trial.
9. Calculate the average number of drops of NaOH used for the 4 trials. Record your result.

Macro-scale Procedures

1. Wash two burets with detergent solution. Rinse them thoroughly, first with tap water, then with distilled water.
2. Obtain about 100 mL of standard base solution in a clean, dry 250 mL beaker. Obtain about the same amount of the acid of unknown concentration in a second 250 mL beaker.
3. Pour about 10 mL of base into one buret and rinse the inside surface of the buret thoroughly. Allow the base to run out the buret tip. Fill the buret to slightly above the 0.0 mL mark with base. Then allow the base to flow out the buret tip until the bottom of the meniscus is at the 0.0 mL mark. Be sure there are no bubbles in the tip. If bubbles are present, add a little more base to the buret and allow it to drain through the tip until it is free of bubbles and the meniscus is at 0.0 mL.
4. Repeat step 3 using the acid solution in the second buret.
5. Place a 125 mL Erlenmeyer flask under the acid buret. Holding a sheet of white paper behind the buret to make the scale easier to read, allow exactly 10.0 mL of acid to flow into the flask.
6. Add exactly 10.0 mL of distilled water to the flask. Using a clean dropper pipet, add three drops of phenolphthalein. Swirl the flask to mix all the ingredients.
7. Place the flask on a sheet of white paper under the buret containing the base solution. To avoid splashing, be sure the tip of the buret is in the flask.
8. Swirling the flask gently, begin the titration by adding NaOH to the flask drop by drop. Continue until a faint pink color remains for about 30 seconds. If "over titration" occurs (the pink color is too deep), follow your teacher's instructions for correcting this condition.
9. Note and record the exact final volume reading on the scale of the base buret. Discard the solution in the flask as instructed. Wash and rinse the flask.
10. Repeat the titration. It is not necessary to refill the burets. Simply read and record the initial volumes of the solutions in the burets carefully.

Skills Development Lab Assessment of Learning:

1. For each trial, calculate the molarity of the HCl solution you were given.
2. To determine the molarity of the HCl solution used, average the results.
3. How reproducible were the results of your two trials?

Invitation to Learn

Now that you can complete an acid/base titration and calculate the molarity of a solution, design and carry out your own experiment to test the acid or base characteristics of a common household material.

Many interesting chemical phenomena can be easily studied with common household products. Reactions between acids (substances with pH below 7.0) and bases (pH above 7.0) can be well illustrated with common products.

Step 1: Choose a household material that contains either acid or base.

Step 2: Design a comparison that involves two similar materials.

Step 3: Based on your knowledge or belief about the two materials, state a hypothesis which you can test.

Step 4: Plan and carry out the test.

Step 5: Report your results accurately. Your final results will be evaluated by your instructor using the method of his/her choice.

Assessment of Learning:

Suggested Evaluation Tools

The formal written lab report is, of course, an acceptable evaluation tool.

Alternative Evaluation Tools

Alternatives to the written report may include the following:

Advertisement:

Construct an advertisement similar to what you would find in a magazine. Use an appeal to logic or authority to support the better product. Include your test results in the advertisement.

Flow Chart:

Construct a flow chart showing the steps you used in your experiment. Include observations in information rectangles. Include conclusions with supporting summary data in a concluding oval.

Info-Commercial:

In a one minute info-commercial, similar to those on television, present your laboratory results. Include a summary of your procedure, a sample of the data obtained and a brief conclusion. Cost comparisons may be added. Practice your info commercial well. It will be presented live in class.

1. How many milliliters of 0.20 M NaOH are required to neutralize 30 mL of 0.50 M HCl?

A. 12 mL
B. 50 mL
C. 75 mL
D. 100 mL

2. A 100 mL sample of hydrobromic acid, HBr, is titrated to an end point with 24.0 mL of 1.5 M NaOH. What is the concentration of HBr?

A. 1.4 M
B. 0.72 M
C. 3.1 M
D. 0.36 M

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This document was submitted for posting to the Internet by the State Science Specialist. Any questions concerning content should be directed to that individual.

Updated September 26 1997 by Michelle Dumas