The Laboratory Assignment: Limiting Reagent (Lab Report Sample)

Limiting Reagent Glendale Community College Department of Chemistry. Version: F2014 1 This laboratory experiment has a formal report. Consult the guidelines for writing a formal laboratory report prior to doing this laboratory experiment so that you know what will be expected of you after the experiment is completed. The formal laboratory reports generally take much longer to type than regular laboratory reports take to write. Post-lab questions should be attached to your report. Introduction The concept of the limiting reagent can be demonstrated by an analogy: Jerry works in the Purchasing Department at a specialty car manufacturing plant in Detroit. The plant gets an order for 100 limited edition cars for Macy’s parade. Jerry purchases 100 car bodies, 100 steering wheels, and 100 tires. Sadly, Jerry gets fired. Do you know why? You probably realized that Jerry did not order enough tires to make the 100 cars required. With his order Jerry could only make 25 cars, and he would have 75 leftover car bodies and steering wheels. In this story the tires are the “limiting reagent” and the bodies and steering wheels are “in excess.” Now let’s apply this concept to chemistry. Consider the balanced combustion reaction of methane gas: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g) According to the reaction one mole of methane gas will react with two moles of oxygen gas to produce one mole of carbon dioxide gas and two moles of steam. Worked example: When a reaction involves a solution, the concentration of the solution is generally reported as molarity (shown in units of molar, M, which is equal to moles of solute per liter of solution)—e.g. a 1.50M sodium chloride solution has 1.50 moles of NaCl per liter of solution or L 1.50 mol NaCl . Given the volume and molarity of any solution, we can calculate the number of moles of solute present. For example, given 50.0 mL of a 1.50M sodium chloride solution, we can calculate the number of moles of NaCl present as follows: 50.0 mL = !" # $% & !" # $% & L 1.50 mol NaCl 1000 mL 1 L 0.0750 mol NaCl What mass of precipitate could be produced if 10.0 mL of 1.500M aluminum chloride reacts with 10.0 mL of 1.500M sodium hydroxide? First, we write the balanced chemical equation: AlCl3(aq) + 3 NaOH(aq) → Al(OH)3(s) + 3 NaCl(aq) The precipitate formed is aluminum hydroxide. In this example we must first calculate moles for each reactant, then continue with the molar ratio step, then convert moles of the precipitate back to grams. Limiting Reagent Glendale Community College Department of Chemistry. Version: F2014 2 10.0 mL !" # $% & !" # $% & L 1.500 mol AlCl 1000 mL 1 L 3 ! !" # $ $% & 3 3 1 mol AlCl 1 mol Al(OH) ! !" # $ $% & 3 3 1 mol Al(OH) 78.0 g Al(OH) = 1.17 g Al(OH)3 10.0 mL !" # $% & !" # $% & L 1.500 mol NaOH 1000 mL 1 L !" # $% & 3 mol NaOH 1 mol Al(OH)3 ! !" # $ $% & 3 3 1 mol Al(OH) 78.0 g Al(OH) = 0.390 g Al(OH)3 Notice that the two product amounts are very different. Even though there is enough aluminum chloride to make 1.17 grams of Al(OH)3 precipitate, there is only enough sodium hydroxide to make 0.390 grams of Al(OH)3. Once that amount of precipitate is made, we run out of sodium hydroxide. Thus, in this example, sodium hydroxide is the limiting reagent, aluminum chloride is in excess, and 0.390 grams of aluminum hydroxide precipitate can be produced. While these calculations may not be as exciting as manufacturing specialty cars, if a chemist miscalculates limiting reagent problems he or she could get fired, cause a serious accident, and (in the extreme case) kill someone due to an explosion or other serious accident! In this lab you will conduct several trials of a reaction, determine the limiting reagent and the reactant in excess for each trial, and record your observations. Limiting Reagent Glendale Community College Department of Chemistry. Version: F2014 3 Procedure: In your Lab Notebook, before beginning the experiment, prepare a data table to record the following data for all six trials (A1, A2, A3, B1, B2 and B3) for each solution, CaCl2 (aq) and Na2CO3 (aq) : initial volume final volume total volume Directly below the data table, label a space to record the exact concentrations of CaCl2 (aq) and Na2CO3 (aq) Part I. Precipitation reactions. 1. Clean and label six test tubes: A1, A2, A3, B1, B2 and B3. In each of the six test tubes, add approximately 4 mL of calcium chloride solution to the six test tubes. Record initial and final volumes for calcium chloride in the Lab Notebook. Also record the exact molarity from the reagent bottle in the Lab Notebook. Calculate total volume delivered for each test tube and record this result in the lab notebook. 2. Now, in test tubes A1 and B1, add approximately 1mL of sodium carbonate solution to each. In each of test tubes A2 and B2, add approximately 4 mL sodium carbonate solution. In each of test tubes A3 and B3, add approximately 6 mL of sodium carbonate solution. Record initial and final volumes for sodium carbonate in the Lab Notebook. Also record the exact molarity from the reagent bottle in the Lab Notebook. Calculate total volume delivered for each test tube and record this result in the lab notebook. 3. Place each test tube in the Vortex Mixer (set to Auto/Touch; adjust the dial to a low-medium setting (a 4-5 setting on the vortex) for about 25-30 seconds. Once all six solutions have been thoroughly mixed, place them in two separate test tube racks, separated by letter (A1, A2 and A3 in one rack and B1, B2 and B3 in a different rack). These two sets will be treated differently. Place the “B” rack in an area on your bench-top where it will not be disturbed. Record the time in your Lab Notebook and do not touch these test tubes (“set B”) until you are ready for Part IV (after at least an hour has passed) so any precipitates can settle well. Part II. Filtration. 1. Obtain three medium-flow filter papers and label them in pencil on their edges with a small “A1”, “A2” and “A3”. Weigh each filter paper and record their masses in your Lab Notebook. Obtain three 50 mL beakers and label them “A1”, “A2” and “A3”. 2. Set up a filtration apparatus with a plastic funnel in a ring stand. Fold and insert the filter paper “A1” into the funnel and place the 50mL beaker “A1” beneath the funnel to catch the filtrate. 3. Pour the contents of test tube A1 into the funnel, using a D.I. wash bottle to wash any remaining precipitate out of the test tube into the filter paper. When the liquid had passed through filter, wash it with about 3mL of D.I. water. When this liquid had passed through the filter, repeated the wash with another 3mL of D.I. water. SAVE the filtrate for each trial to use in Part III. Limiting Reagent Glendale Community College Department of Chemistry. Version: F2014 4 4. Remove the filter paper and carefully open it up, placing it on a watch glass. Place the watch glass on a hot plate (set to heat setting 4, or 70°C) to dry. Record the time drying started and ended in your notebook under observations. While the filter paper “A1” is drying, repeat steps 2-4 above with “A2”. While the filter paper “A2” is drying, repeat steps 2-4 with “A3”. (You may filter all three simultaneously with three ring clamps, three funnels, and three beakers if you wish. Please dry each on a separate hot plate, though.) 5. Weigh each filter paper when dry and record its new mass in your Lab Notebook. Calculate the actual yield of precipitate for each trial and record this in your Lab Notebook. 6. Write and balance the chemical equation for the reaction in your Lab Notebook. Identify the precipitate in this reaction. Calculate the theoretical yield of each of these three trials, and then determine the percent yield for each of these trials. Determine the limiting reagent and the reagent in excess for each trial. Be sure to clearly label each trial’s calculation and results. Results will be graded in your lab notebook! Part III. Testing the Filtrates from Set A. 1. Test the ions dissolved in the filtrate solutions by transferring a small amount of the solution from filtrate trial A1 to two different clean test tubes with a disposable pipette. Use new disposable plastic pipettes for each test to prevent cross-contamination. 2. Add about 1 mL more CaCl2 (aq) solution to the first new test tube and about 1 mL more Na2CO3 (aq) to the second. Record your observations in your notebook in Data & Results Summary Table – Part III (see page 5 as a reference). 3. Repeat steps 1-2 for the filtrates from A2 and A3. Remember: use new pipettes for each test. Part IV. Observing the Precipitates in Set B. 1. Now, without disturbing the precipitates, examine the test tubes in test tube rack B that were set aside for at least an hour. Draw the three test tubes in your Lab Notebook, making sure to note the level of the precipitates and the level of the liquid surface in each. Observe and record the physical properties of the precipitates. Finally, complete the Data Page and Postlab Questions, pages 6 -7. Use the Format for Formal Lab Report on the CHM151LL website to help you write your Formal Report. Combine your work in the following order and staple together to submit the following week: Formal Lab Report, copies of your Lab Notebook pages, and the post-lab question sheets. Limiting Reagent Glendale Community College Department of Chemistry. Version: F2014 5 Please copy the following Data and Results Summary Tables in your lab notebook. Your data should be neatly entered into your lab notebook as the experiment is carried out, not on this page. Example Data & Results Summary Table – Parts I & II. Trial A1 A2 A3 B1 B2 B3 Volume of CaCl2 (aq) Volume of Na2CO3 (aq) Theoretical mass of precipitate Limiting reagent Reagent in excess Actual mass of precipitate Percent yield Example Data & Results Summary Table – Part III. Trial A1 A2 A3 Predicted reagent in excess Observation after adding CaCl2 (aq) Observation after adding Na2CO3 (aq) Limiting Reagent Glendale Community College Department of Chemistry. Version: F2014 6 Postlab Questions. Name: ________________________ Partners: ______________ _________ Section Number: ________________ Turn in pages 6-7 along with the formal report and pages from your lab notebook 1. Betty Smith is making little stick figure donkeys for her daughter’s birthday party. Each donkey needs one head, one body, two ears, and four legs. Draw a stick figure donkey here à How many donkeys can she make with 18 heads, 20 bodies, 39 ears and 74 legs? The limiting body part is The body parts in excess are How many of each excess piece will be left over? 2. Consider the reaction, N2 (g) + 3H2 (g) à 2NH3 (g) . The picture to the right represents a mixture of N2 (g) and H2 (g) just before reaction occurs. (Smaller lighter spheres are hydrogen molecules, and larger darker spheres are nitrogen molecules.) What is the limiting reactant? How much of the excess reactant remains after the reaction is complete? 3. Consider the following reaction: V2O5 (s) + 2NaOH (l) Δ ""→ 2NaVO3 (s) + H2O (g) red colorless yellow colorless solid liquid solid gas When the red solid is mixed with the colorless liquid and heated, the mixture starts to give off a gas and change color. When the reaction has stopped, the result is a dry, yellow solid containing small, red particles. Which substance is the limiting reactant? Explain how you determined this. Limiting Reagent Glendale Community College Department of Chemistry. Version: F2014 7 4. In the beakers below, using your calculation results, draw what the filtrate solutions contained for each trial. Show the precipitate and the excess reagents in the beakers. (Recall how we showed solution contents in the pictures from the Ionic Solubility Lab.) Trial A1 Trial A2 Trial A3 5. If calcium chloride is the limiting reagent and sodium carbonate is in excess: a. Which ions are present in the solution above the precipitate? _______________ b. What would happen if you added CaCl2 (aq) to the solution? c. What would happen if you added Na2CO3 (aq) to the solution? 6. If sodium carbonate is the limiting reagent and calcium chloride is in excess: a. Which ions are present in the solution above the precipitate? _______________ b. What would happen if you added CaCl2 (aq) to the solution? c. What would happen if you added Na2CO3 (aq) to the solution? 7. In the space below, neatly draw three identical, empty test tubes. Label the test tubes “B1”, “B2” and “B3”. In the three test tubes, draw a horizontal line to show the level of the liquid and another horizontal line to show the amount of precipitate produced in lab as the volume of sodium carbonate increased with each trial.