BASIC VERSION

Background

You are a research scientist working with the Department of Agriculture. A farmer from a nearby ranch has brought a problem to you. In the past 10 years, his hens’ eggs have become increasingly fragile. Many of them have been breaking and he is beginning to lose money on the operation. The farmer believes his problems are linked to a landfill upstream, which is being investigated for illegal dumping of PCBs and other hazardous chemicals.

Birds have a chemical process that allows them to rapidly produce the calcium carbonate required for eggshell formation. As you know, this shell provides protection for the developing embryo. Research has shown that PCBs can decrease the amount of calcium carbonate in the eggshell, resulting in shells that are more fragile.

Tests have been done on the farmer’s eggs that have shown their shells are 78% calcium carbonate. Your job is to determine the percent composition of calcium carbonate in shells from other local farms not located downstream from the landfill.

You will be reacting the eggshell with 1M HCl. After the reaction, you will perform a titration with 1M NaOH. When performing a titration, you take a known concentration of NaOH and add it to a solution of HCl. For the HCl, you know the amount of solution that you have but you do not know the concentration (so you do not know the number of moles HCl present). You very carefully measure how much NaOH you add until you reach the equivalency point. This is the point at which the H⁺ ions are equal to the OH^- ions. But how will you know when you reach that point? Since these two ions are equal in amount, that means that the pH will be a neutral 7. You will use an indicator that will change colors when the pH goes from an acidic pH (less than 7) to a neutral pH of 7. Once you figure out how much of the HCl is left, you can determine how much of the HCl reacted and use this information to determine the percent composition of calcium carbonate in the eggshell.

Pre-lab Questions

1. What is the balanced chemical reaction that occurs between HCl and calcium carbonate?
2. What is the balanced chemical reaction that occurs between HCl and NaOH?
3. What does the equivalency point mean in a titration?
4. What is the mole ratio of HCl to NaOH?

Problem

What is the percent composition of calcium carbonate in the eggshells from nearby farms?

Materials

• 2 burets
• 2 funnels
• 1 250 ml beaker
• 50 ml of 1M HCl
• 50 ml of 1M NaOH
• Bromothymol blue indicator
• Electronic scale
• Mortar and pestle

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Wash your hands thoroughly before leaving the lab.
• Follow instructions for clean-up of materials and disposal of any chemicals.

• When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
• When diluting acids, always add acid to water.

Procedure

1. Obtain a sample of eggshell and crush it as fine as possible with the mortar and pestle.
2. Place a 250ml beaker on an electronic balance and record its mass.
3. Place between 1 to 2 grams of crushed eggshell into the beaker. Record the exact mass of eggshell you used in the table below.
4. Using a funnel, fill a burette with about 50 ml of HCl.
5. Add about 50 ml of 1M HCl. Record exactly how much you used in the table below. The solution will bubble up and you will need to stir it to make sure all of the eggshell reacts.
6. Using a separate funnel, add about 50 ml of NaOH to the second burette. Record the initial volume of NaOH in the burette.
7. Add 5-7 drops of bromothymol blue to the HCl/eggshell solution. The solution should be a pale yellow (you may want to place a white piece of paper under the beaker so you can see the color change clearly)
8. Slowly add NaOH to the HCl/eggshell solution, swirling or stirring as you do. When you see the color change with 1 drop of NaOH and then go back to yellow after stirring, you are close to the end point. Add drop by drop at this point.
9. Once the solution goes to green, you are at the end point. Record the final amount of NaOH in the burette.
10. Follow your teacher’s instructions for cleaning your lab equipment.

Data

Calculations

1. What was the initial volume of HCl that you used?
2. Calculate the amount of moles of HCl that you had initially.
   (reminder: moles = Molarity x Volume…and volume should be in Liters!)
3. What is the volume of NaOH you added?
4. How many moles of NaOH did you add?
5. At equivalency point, how many moles of HCl reacted with the NaOH?
6. Subtract the number of moles of HCl that reacted with the NaOH from the initial number of moles of HCl. This is the amount of HCl that reacted with the calcium carbonate in the eggshell.
7. Now do stoichiometry to determine how much calcium carbonate it would take to react with the amount of HCl you calculated in the previous question.
8. Use the amount of calcium carbonate you found to determine the percent of the total eggshell that is calcium carbonate.

Analysis and Conclusions

1. Was your percent composition higher or lower than the percent composition that the farmer found?
2. Do you think that the farmer is experiencing fragile eggs because of a lack of calcium carbonate in his eggs?
3. Where do you think there might have been a source of error in your experiment and how might you correct that source of error if you did the experiment again?
4. Workers in another city tested eggshells and found that a normal eggshell is about 97% calcium carbonate. Calculate the percent error for what you found.

ADVANCED/INQUIRY VERSION

Background

You are a research scientist working with the Department of Agriculture. A farmer from a nearby ranch has brought a problem to you. In the past 10 years, his hens’ eggs have become increasingly fragile. Many of them have been breaking and he is beginning to lose money on the operation. The farmer believes his problems are linked to a landfill upstream, which is being investigated for illegal dumping of PCBs and other hazardous chemicals. Your job is to find if the PCBs are the cause of the hens’ fragile eggs.

Birds have a chemical process that allows them to rapidly produce the calcium carbonate required for eggshell formation. As you know, this shell provides protection for the developing embryo. Research has shown that PCBs can decrease the amount of calcium carbonate in the eggshell, resulting in shells that are more fragile.

Tests have been done on the farmer’s eggs that have shown their shells are 78% calcium carbonate. Your job is to determine the percent composition of calcium carbonate in shells from other local farms not located downstream from the landfill.

In order to fully support your findings, you need to determine the calcium carbonate percentage using two different methods.

In the course of completing your analysis you should make use of all of the following topics that we have covered this semester:

• Percent composition
• Dimensional Analysis
• Stoichiometry
• Gas Laws
• Gas Laws Stoichiometry
• Molarity
• Acid-Base Neutralization

Materials

You will have access to:

Procedure

List the procedure you will follow in a very organized manner and advise the supervisor if other materials will be needed.
Carry out your procedures after approval.

Data

Record your data in a VERY NEAT, LABELED, and ORGANIZED table. If there is time and enough available material you might consider carrying out your procedure a second time.

Analysis

1. What are the two balanced chemical equations you used to accomplish your task? Identify the type of chemical reaction for each of them.
2. What percent composition of calcium carbonate did you find with your two different methods? SHOW ALL OF YOUR CALCULATIONS IN A VERY ORGANIZED AND EASY TO FOLLOW MANNER!!!!!
3. Which method do you believe is more reliable for determining the amount of calcium carbonate in an egg and why?
4. Workers in another city tested eggshells and found that a normal eggshell is about 97% calcium carbonate. Calculate the percent error for what you found.

Overall

Write a report for the supervisor detailing the purpose of your procedure, your hypothesis, a materials list, safety concerns, experimental design (independent and dependent variables) detailed procedure (keep in mind you might have two procedures with 2 of each of the above depending on how you present it), your neat, labeled and organized data table, all of your calculations in a very neat and organized manner and your analysis of the data (including a discussion of sources of error).