Stoichiometry Lab Purpose The purpose of this lab is to

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Stoichiometry Lab

Purpose The purpose of this lab is to predict the theoretical yield of a product of a chemical reaction using stoichiometric calculations, then compare the theoretical yield of the product to the actual yield when the reaction is done in the lab.

Background Information The reaction we will be studying is the compound sodium carbonate reacting with hydrochloric acid to yield sodium chloride, water, and carbon dioxide. The unbalanced equation for this reaction is: Na2CO3(s) + HCl(aq)  NaCl(aq) + H2O(l) + CO2(g) NaCl is the product on which we will be focusing.

General Procedure 1. Cut and tape these instructions and questions into your lab book. 2. Carefully read the entire procedure with your group, and make a data table in your lab book. Every member of the group must have a data table. The information you will need to record in the data table is detailed in the procedure. 3. Clean a 50 mL Erlenmeyer flask by scrubbing with soap and a scrub brush, rinsing with tap water, then rinsing well with distilled water. Distilled water is in the squeeze bottles. 4. Set up a ring stand over a Bunsen burner. Place wire gauze over the ring, and place the clean Erlenmeyer flask on the ring. Heat the flask over the flam, until it is completely dry. Carefully, using tongs (the flask will be hot enough to burn you!), place the flask on the triple beam balance and record the mass of the flask. 5. When you are ready, I will place a sample of sodium carbonate in the flask. Record the mass of the flask plus the sample. 6. Calculate the mass of the sample, and include in the data table. 7. Slowly, using the dropper, add hydrochloric acid to the sodium carbonate. Record your observations. Keep adding the hydrochloric acid until the reaction stops. Use a toothpick to stir the solution, then add a few more drops of hydrochloric acid. Keep stirring and adding hydrochloric acid until the reaction

completely stops. Add a little extra hydrochloric acid and stir. Finally rinse the toothpick into the beaker with hydrochloric acid. Wait a few more moments to make sure the reaction is completely done. (At this point, you will probably have to stop and continue the next day.) 8. If you have to stop and continue the next day, cover your flask and place it on the blue tray in the cart on top of a piece of paper with your period and names on the paper. DO NOT use tape or write on the beaker. 9. The next day, uncover the flask and place it back onto the wire gauze on the ring stand and heat with the Bunsen burner. Slowly, carefully boil off all the liquid until only NaCl is left in the flask. Watch so that none of the liquid boils or bubbles out of the flask. Remove the burner and allow the flask to cool slightly if you need to slow down the boiling, then lower the flame. 10. Once all the liquid has boiled off, use the tongs to place the flask on the triple beam balance and measure and record the mass of the Erlenmeyer flask plus the sodium chloride. 11. Calculate the mass of the sodium chloride in the flask and record in the data table. Safety Considerations 1. Safety goggles and lab aprons must be worn at all times. 2. Be careful around the flame of the Bunsen burner. Turn off the gas when not in use. If the Bunsen burner gets knocked over, immediately turn off the gas. Assign one person in each group to always watch the flame and everyone around the flame. It is this person’s job to also be aware that the ring stand and beaker may also be hot. It is this person’s job to be ready to turn off the gas in an emergency and to be sure the gas is turned off when finished. 3. The beaker will be hot enough to burn after you are done heating it. Use tongs to pick up the hot beaker. 4. Tie back long hair. No dangling sleeves or jewelry around the flame. 5. Sodium Carbonate is irritating to the skin and eyes. Avoid getting on your skin. Wash with soap and water if you do. Immediately flush eyes in eye wash station if you get any in your eyes. 6. The hydrochloric acid we are using is strong enough to burn skin and eyes. If any splashes on you, immediately flush your skin with water. Immediately flush eyes in eye wash station if you get any in your eyes. Avoid getting HCl on your clothes, or you may have an unpleasant surprise the next time you wash your clothes. 7. Avoid inhaling the vapors when heating. 8. After you are done cleaning up, wash your hands thoroughly with soap and water. 9. All reactants and products can be washed down the sink with large amounts of water.

Some Reminders 1. All mass measurements should be to the nearest hundredth of a gram. The hundredth digit will have to be estimated. 2. As you perform the lab, you must be taking detailed observations. There are observations that can be taken during almost every step of the procedure. These observations can include any difficulties or problems you may have encountered, including how you solved them, evidence that a chemical reaction is taking place, anything you do that is not part of the procedure I gave you, descriptions of the physical properties of the reactants and products, etc. 3. It is very important that you make your measurements as accurately and precisely as possible. Your results can only be as good as your measurements. 4. You should also be making a materials list as you do the lab. 5. All data should be recorded in a data table in your lab books the instant it is taken. All members of the group should be taking the same data. Remember, you can rewrite this data table later if you want. I will take off points from everybody in the lab group if I catch anybody writing any data in any location other than a premade data table in your lab books. 6. When using a Bunsen burner, be sure the flame is blue, and not yellow (see picture on page 384 in textbook). Ask for help if you cannot get your flame to be the proper color. 7. Be sure you go back and write a title for your lab. 8. You will need the following sections in the lab write up: I. Handouts II. Title (“Stoichiometry Lab” is not a very good title, because it doesn’t contain any information about what we actually did, plus you need to come up with your own title, not use mine) III. Data Table V. Materials List VI. Observations IX. Analysis (This consists of the following questions answered in complete sentences, including all calculations you do to get the answers. Be sure to show all your work, and include units in your calculations)

Questions to Answer (Be sure to show all work, and answer in complete sentences!) 1. Balance the equation for the reaction used in this lab. 2. What amount (in grams) of sodium carbonate did you start with? Explain how you calculated this amount from the data in your data table. 3. Use the amount of sodium carbonate you started with and the balanced equation to calculate the amount of NaCl (in grams) that should be produced. (The amount of NaCl that should be produced based on your calculations is called the “Theoretical Yield.”) Show all your work. What is your Theoretical Yield? 4. The actual amount of NaCl you end up with is called the “Actual Yield”. What is your actual yield? Explain how you got this amount from the data on your data table. 5. Use the handout titled “Percent Yield” to calculate your percent yield. Show all your work. What was your Percent Yield? 6. In a perfect world, your percent yield would be 100%, but this is very unlikely. Explain all the possible reasons your percent yield might be greater or less than 100%. The more reasons you can come up with, the better. Be specific in your reasons. For example, don’t say “some of the products may have been lost.” Explain exactly how and in what step the products may have been lost. 7. What does it mean if a percent yield is less than 100%? 8. What does it mean if a percent yield is greater than 100%? 9. The lab was designed so that the hydrochloric acid does not limit the amount of product produced. What did we do in this lab to make sure that more than enough hydrochloric acid was used? 10. When you added the hydrochloric acid to the sodium carbonate, a chemical reaction took place. What evidence did you see that a chemical reaction took place? 11. If you did the lab correctly, the mass of the H2O produced would not be included in your measurements. What did you do to make sure that the mass of the H2O was not included in your measurements? 12. Why would the mass of the CO2 produced not be included in your measurements?

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