## Theoretical vs. Actual Yield

Objective
The reaction of calcium carbonate with hydrochloric acid to produce water, calcium chloride and carbon dioxide will be studied. The loss of mass due to the escape of carbon dioxide gas will be predicted based on the starting mass of calcium carbonate and the chemical equation:
CaCO3 + 2HCl --> H2O + CaCl2 +CO2
This prediction will be tested by comparing a measurement of the mass of the combined reactants before and after the reaction. The percent yield will be calculated and reasons for its failure to be 100% will be investigated.

Materials
 1. 300 mL or 500 mL Erlenmeyer flask 5. lab balance 2. approx. 35 g CaCO3 6. stirring rod 3. 150 mL 6 molar HCl 7. lab notebook 4. a watch glass 8. safety gear

Pre-Lab Questions
Write your answers to these questions neatly on a separate piece of paper, showing all steps of all calculations. Be sure to do the calculations before the experiment and to re-write them if they aren’t neat and easy to read.
1. What is the molar mass of each reactant and product?
2. What is the most important molar ratio this experiment will be concerned with?
3. How many moles of calcium carbonate are represented by 35 g?
4. One hundred fifty milliliters of six molar hydrochloric acid solution represents 0.9 mol (take my word for it). Which reactant is in excess? Which is the limiting reactant? By how much (in moles) does the excess reactant exceed the limiting reactant?
5. Based on your answers to the previous question, how many moles of carbon dioxide do you expect the reaction to produce?
6. How many grams is that?
7. Say you measure a loss of only 7.5 g (meaning that 7.5 g of carbon dioxide escaped). What would the percent yield be? (Remember to convert to moles first!)
Safety Note: Six molar hydrochloric acid has six moles of acid per liter of solution. This is strong enough to cause serious burns and irritation of the skin. If contact with the skin is made immediately rinse with plenty of water and neutralize with baking soda, if possible.
Procedure
• Fair Warning: you will have to take notes, make your own data tables, and (to a certain extent) decide what is important enough to record!
• Make sure your Erlenmeyer is clean and dry and find its mass using your balance
• Remove the flask from the balance and measure as close to 150 mL of 6M HCl as possible; add the acid to the flask and find the new mass
• Measure out approximately 35 g of CaCO3 chips onto a watch glass and carefully note its exact mass
• Remove the flask from the balance again and add the chips, careful not to splash any of the acid!
• Place the flask on the balance again to watch the mass decrease as the CO2 is produced; the reaction should produce plenty of bubbles
• Stir and swirl the mixture from time to time for at least ten minutes or until no more bubbles are produced
• Hurry up and wait because you need to make sure that the mass does not decrease any further; stir occasionally for a few more minutes until you are sure the mass is holding steady
Discussion
Write a lab report on a separate piece of paper and be sure to create and include a neat, readable data table. Keep this lab for future reference. The lab report should include the following sections:
Abstract: Briefly (50 words or fewer) state the purpose and findings of the lab. Free advice (isn’t it always?): do not write the abstract until you have written the rest of the report and done all of your calculations since you need to report your results in the abstract, anyway.
Materials: Write a list of the materials used in the lab.
Procedure: Write the procedure given in your own words.
Data: You will collect two kinds of data: qualitative and quantitative. The qualitative data include your own impressions and observations about the experiment and may include pictures. The quantitative data are the data you were told to collect in the procedure. Also include any other numerical data that is relevant.
Data Analysis: Calculate the actual yield of carbon dioxide. Find the percent yield using the theoretical yield you calculated in the pre-lab questions. Show all work clearly.
Error Analysis: be sure to mention how possible errors may affect results. Do not simply say that ‘human error’ probably caused problems in the data. That is a given. Try to think of what could be changed in the experimental design to ensure better data. For instance, would a sample of calcium carbonate that was quite a bit finer have given better results? Why?
Conclusion: Briefly state your results. No new information should be introduced here.