7# 43CJ ''zx 9[*[O [[[[[[[[Experiment III The Reaction of Calcium Chloride with Carbonate Salts PRE-LAB ASSIGNMENT Reading: Before coming to your discussion section, read the following: 1. Section 3-4 and 3-6 in Olmstead and Williams. 2. The remainder of this experiment in this manual. Pre-lab assignment to hand in: Prepare written responses to the following questions and submit your work to your instructor at the beginning of your discussion period: 1. What should be placed in your wash bottle for this experiment? 2. Give the formulas for two compounds formed from the magnesium ion, Mg2+, the hydrogen ion, H+, and the phosphate ion, PO43- (all three ions should appear in both formulas). 3. Predict the formula of the salts prepared from the reaction of CaCl2 and Na2CO3. Use sections 3-3 and 3-4 in Olmstead and Williams to help you identify the ions that will react. INTRODUCTION The purpose of this lab is to help you discover the relationships among the reacting species and products in a chemical reaction. In this lab you will react a calcium chloride solution with either lithium carbonate, sodium carbonate, or potassium carbonate. The precipitate that results will be filtered and weighed. In each determination you will use the same amount of calcium chloride and differing amounts of a carbonate salt. This experiment is a "discovery"-type experiment. The procedure will be carefully described, but the analysis of the data is left purposely vague. You will work in small groups to decide how best to work up the data. In the process you will have the chance to discover some principles, to use what you have learned in lecture, and to practice thinking about manipulative details and theory at the same time. Plotting your data in an appropriate manner should verify the identity of the precipitate and clarify the relationship between the amount of carbonate salt and the yield of precipitate. Predicting the formulas of ionic compounds. Compounds like calcium chloride, CaCl2, and sodium carbonate, Na2CO3, are ionic substances. Ionic substances dissociate in aqueous solution to form ions: CaCl2 (aq) = Ca2+(aq) + 2 Cl-(aq) A list of common ions is given in your text and referenced above. A short excerpt is tabulated below: Some Common Ions Cations Anions H+ hydrogen ion OH- hydroxide ion Na+ sodium ion Cl- chloride ion K+ potassium ion CO32- carbonate ion Ca2+ calcium ion NO3- nitrate ion Ag+ silver ion PO43- phosphate ion NH4+ ammonium ion SO42- sulfate ion This table can be used to predict the formulas of ionic compounds. The ions combine to give electrically neutral compounds. For example, the combination of K+ and PO43- would give K3PO4, which has a net charge of zero. If a complex ion such as nitrate ion occurs more than once, it is enclosed in parentheses: Ca(NO3)2. Predicting the product of precipitation reactions. Sometimes when two water soluble ionic compounds are mixed in aqueous solution a precipitate results. For example, a precipitate is formed when silver nitrate and sodium chloride are mixed in solution. How would you predict the identity of the precipitate? This can be done in two steps. Step one: break the parent compounds into their respective ions, and step two: exchange the ions between partners to predict the products:  Therefore, the precipitate could be either AgCl, NaNO3, or both. Later in this course you will learn how to predict which ionic substances will be insoluble. In this example, AgCl is insoluble in water and precipitates out of solution, while the Na+ and NO3- ions remain uncombined in solution. This reaction would then have the balanced equation: AgNO3(aq) + NaCl (aq) = AgCl (s) + Na+(aq) + NO3-(aq) or the net ionic equation: Ag+(aq) + Cl-(aq) = AgCl(s) The general rules for water solubility of common ionic compounds include the following two rules, with rule one taking precedence over rule two: rule1. All common compounds of the alkali metals (Group IA) and the ammonium ion (NH4+) are soluble. rule 2. Almost all carbonates, phosphates and hydroxides are insoluble. Therefore, MgCO3, Ag3PO4 and Ca(OH)2 are predicted to be insoluble in water, but Na2CO3 or (NH4)3PO4 or KOH is predicted to be soluble. Outline of this experiment. You will analyze two different samples and share your data with your group. At the end of the lab, the data from the entire class will be combined. For each sample you will react the same amount of calcium chloride solution with two different amounts of a carbonate salt. Your instructor will specify which carbonate salt and the two weight ranges to use. The choice of carbonate salts will be from lithium, sodium, and potassium carbonate. The following procedure will be used for each sample: 1. Weigh a clean dry Gooch crucible. The Gooch crucible will be used to collect your precipitate. 2. Dissolve a precisely weighed amount of your carbonate salt in 50 mL 0.05M ammonia. 3. Heat the solution almost to boiling. 4. Add 10mL of the CaCl2 solution. 5. Continue heating and stirring for two minutes. 6. Cool the solution. 7. Collect the precipitate in a Gooch crucible. 8. Dry the precipitate in the Gooch crucible in an oven at 160oC. 9. Weigh the precipitate and the crucible. The difference in weight between steps 9 and 1 gives the weight of the precipitate. PROCEDURE 1. Label two Gooch crucibles. Weigh these crucibles to an accuracy of 0.5 mg (i.e. 0.0005 g). This step can be completed at any time before step 7, so plan your work to use time efficiently. To the accuracy of this experiment it is permissible to use your fingers to handle the crucibles. 2. Label two weighing bottles and use them to weigh out two samples of the carbonate salt assigned to you by your instructor. Record your weights to an accuracy of 0.5 mg. If your instructor assigns the weight range of 0.30 to 0.45 g for one of your samples, any weight within this range is fine, but, whatever the weight, you must know the result to 0.5 mg. For example, 0.4255 g would be within the range and to the required accuracy. 3. Your instructor will assign the weight range for each of your samples from the table below. Weight Ranges for Carbonate Samples 1 2 3 4 5 6 7 Li2CO3 0.04-0.06 0.06-0.10 0.10-0.14 0.16-0.20 0.20-0.22 0.22-0.24 0.25-0.30 Na2CO3 0.06-0.10 0.10-0.14 0.16-0.20 0.22-0.26 0.26-0.30 0.30-0.34 0.35-0.40 K2CO3 0.08-0.12 0.12-0.18 0.20-0.26 0.28-0.34 0.34-0.38 0.38-0.42 0.43-0.50 You will quantitatively transfer your carbonate salts to two clean, labeled 150 mL beakers, making sure you record which sample goes into which beaker. To transfer the salt, empty the contents of the weighing bottle carefully into the beaker. Measure 50 mL of 0.05M ammonia into a graduated cylinder, and rinse the weighing bottle with small portions of this solution, adding these rinses to the beaker; use all 50 mL of ammonia (Ammonia is used instead of distilled water to make your solution slightly basic. A basic solution will minimize any loss of the precipitate you will be creating in this experiment.). Stir the solution with a glass stirring rod until all of the solid dissolves. Do not remove the stirring rod once you have placed it into the beaker; you do not want to lose any of the solution. If you have difficulty dissolving the salt, there is a sonic bath available for your use. Simply place the beaker into the bath for a few minutes until dissolution is complete. (The beaker should not touch the walls or bottom of the sonic bath.) Heat the beakers on hot plates until water vapor begins to condense on the sides of the beaker. Do not allow the solution to boil. 4. Dispense 10.00 mL of a solution of CaCl2 directly into each beaker. The CaCl2 is in dispenser bottles which can be set to deliver a precise volume of solution. The concentration of CaCl2 will be approximately 22.2g/L, but the exact concentration will be on the label. Be sure to write the exact concentration in your notebook. 5. Stir each solution occasionally and continue to heat for two minutes. 6. Allow the beakers to cool until they are at room temperature. Cooling can be hastened by placing the beakers into an ice-water bath for a few minutes. 7. Filter each precipitate into a weighed Gooch crucible. The filtration apparatus is shown in Figure 1. When pouring the supernatant from the sample beaker, pour down the stirring rod to avoid spilling, as shown in Figure 2.  Figure 1. Filtration apparatus Figure 2. Pouring down a stirring rod. Use your wash bottle containing 0.05M ammonia to rinse the precipitate into the Gooch crucible, as shown in Figure 3. Rinse the sample beaker three times, to ensure complete transfer.  Figure 3. Washing the precipitate into the Gooch crucible. 8. Place the Gooch crucibles on their sides in half of a Petri dish . Label the Petri dish so that it is easy to recognize. Place the Petri dish and crucibles in the oven at 150-160oC for 30 minutes. While you wait for the samples to dry, set up an EXCEL spreadsheet to accept your data for plotting. 9. Remove the Petri dish and crucibles from the oven and allow them to cool to almost room temperature ( remember hot objects appear lighter than they should in an analytical balance because of air currents which lift the balance pan). Weigh the crucibles. 10. Procedure for cleaning sintered glass crucibles: a) Leave suction filtration apparatus set up, but dump out the ammonia in the flask. b) Scrape as much of the precipitate as possible into the waste container provided. c) Wash the crucibles with water, using a brush to remove more solid. d) Return the crucibles to the suction filtration apparatus and pour in some 3M HCl (with suction off). After fizzing stops, attach hose and remove HCl. Repeat. e) Allow distilled water to pass through the crucibles to rinse away the HCl. Calculations First calculate the weight of CaCl2 in each of your samples. For example, assume that the concentration of the stock solution was given as 22.26 g L-1 CaCl2 in water. Then 10.00 mL ( i.e. 0.01000 L) of this solution would contain ( 0.01000 L ) ( 22.26 g L-1 ) = 0.2226 g of CaCl2. Share your data in groups of four. Make sure each member of your group used the same carbonate salt. For a preliminary graph, plot the weight of precipitate for each run on the vertical axis and the weight of the alkali-metal carbonate salt on the horizontal axis. For the rest of your calculations, your group is on its own. Here are some hints on how you might proceed. 1. Consult with your lab instructor often, but don't be frustrated if your instructor doesn't answer all of your questions or asks a question in return. You will have a better sense of accomplishment if you work out the details on your own. You will hopefully gain a better grasp of the theory of this experiment if you try many ideas, some of which might be wrong. 2. Try to come up with a way of plotting the data that makes the underlying relationships as clear as possible. (Yes, we know this is the second lab of this course and you don't know much chemistry yet. Have fun trying different ideas. The spreadsheet will help speed calculations.) 3. Borrow data from nearby groups that used other salts, and try out your ideas. At first the data from different carbonate salts will look different. But, the way you plot your data can resolve these differences. 4. Chemical theories are often judged on the basis of how well they unify and explain data from different experiments. See if your plots can do that. Try drawing straight lines through your data, but don't try to fit a straight line to all the data points! Why? LABORATORY NOTEBOOK Keep track of all weighings in your lab notebook. Record all observations in your lab notebook. Remember to include units on all numbers that have them. All members of the group should keep track of the various calculations that you tried, in your notebooks. Copy and tape printouts of your calculations into your lab notebooks. All members of the group should have a copy of the final graph taped into their notebooks. REPORT Your report will contain only the answers to the questions below. 1. Give the weight of CaCl2 that you used. (Remember: always show calculations). Give the identity and weights of the carbonate samples that you used and the corresponding precipitate weights. 2. Give the names of the other three members of your group. All group members should attach a copy of the final graph that your group produced. 3. For the linear curve fitting that you did (part 4 in the calculations), give the value of the slope with appropriate units, and in a sentence or two explain the meaning of the slope. 4. What is the identity of the precipitate? What experimental evidence do you have that this is true? Acknowledgements: This laboratory was inspired by a similar lab at Holy Cross and J.M. DeMoura, J.A. Marcello, J. Chem. Ed., 64 (5), 452 (1987). CH 141 Lab: Expt. III -- -- CH 141 Lab: Expt. 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