HONORS CHEMISTRY THE LIMITING REACTANT JOB S METHOD OF CONTINUOUS VARIATION

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1 HONORS CHEMISTRY THE LIMITING REACTANT JOB S METHOD OF CONTINUOUS VARIATION PURPOSE: Determine through stoichiometry which reactant is in excess. Practice data analysis through graphical methods. Confirm the formula of a precipitate. Confirm through lab data the charge on the nickel (II) ion and the hydroxide ion. Develop skills for determining the mole fraction for ions in solution. DISCUSSION: Job s Method of Continuous Variation keeps the total number of moles of reactants constant throughout a series of mixtures of reactants. The fraction of the moles of the reactants, however, are varied from one mixture to another. We will systemically decrease the amount of one reactant and increase the amount of the other. Because the maximum change will occur when the mole fraction of the reactants is closest to the actual stoichiometric mole ratio, both the formula of the product and the reactant stoichiometry can be determined by using this approach. The above paragraph really sounds complicated! It s really not that bad once we consider how we are going to do it. Both solutions, NiCl 2 and NaOH, will be the same concentration. We will keep the total volume constant when we mix the two but change the amount of NiCl 2 and the NaOH as we go from one sample to another. There are several ways to use this method that are valid. To conserve time, however, we will concentrate on only one of these methods. Today, we will be looking at the intensity of the color of the superannuate liquid. We would have a little trouble telling the intensity the color of the mixtures if we just try to tell using the detectors that are on either side of our nose. We are going to get some help using an instrument called a spectrophotometer. That instrument is just a fancy way of measuring the intensity of a light source. You know RT, he has to tell you a bit about the way that instrument works. The spectrophotometer is an instrument that measures a fraction (I/I o ). Oh boy! With the ratio we are comparing the intensity of two light beams. I o is how bright the light beam is that is going into the sample and I tells us how bright the beam is that leaves the sample. Remember, white light is a mixture of all the colors of the spectrum. If you look at red Gator Aid, it only makes sense that certain colors are filtered out by that liquid. Let s keep thinking about the Gator Aid. If you have a 2 gallon container of it, that amount of liquid will appear darker in color than a juice glass of it. How much of the solution the light passes through affect the amount of light absorbed. There is another factor that we need to consider. If you take two juice glasses of our Gator Aid but one is half water and the other is full strength, the colors will be different. These ideas now become exciting to a chemist! The amount of light absorbed depends upon: 1) the concentration of the sample (How much water has been used to dilute it.) 2) The path length of the light through the sample (Are we talking 2 gallons or a juice glass?) and 3) the wavelength of the light (the color of the light). If we hold the amount of sample and the wavelength constant, we can play with our ratio of I/I o, and find a mathematical relationship to concentration. The spectrophotometer is our handy tool that will helps us find that relationship. We will use small test tubes called cuvettes. They are made so that the light will pass through the same thickness of solution. The spectrophotometer will sort out the color of light we want and will tell us how intense the light is that is passing through our 55

2 sample. We now have information that will allow us to find the concentration of our solutions. How cool is that! Let s collect some data! PRE-LAB QUESITONS: 1) How much precipitate will you get if only nickel (II) chloride or sodium hydroxide are used? 2) Write the balanced chemical equation for the reaction that will take place between Nickel (II) chloride and sodium hydroxide in this lab. 3) What is the mole ratio of sodium hydroxide and Nickel (II) chloride taken from the equation? 4) If you are given the molar concentration of a solution and the volume of that solution that you used, how would you find the moles of solute that you measured out? 5) Set up a Data Table #1. It needs to include the following columns: Test Tube #, Volume of NiCl 2, Moles of NiCl 2, Volume of NaOH, Moles of NaOH, Mole ratio,, Color of superannuate liquid/observations. This data table need to have 17 rows for data. If you format the table in the landscape arrangement, it may work well. 6) Think of our red Gator Aid again. Also think of the visible spectrum, ROYGBIV. Which colors are being filtered out? Which colors can pass through the Gator Aid? PROCEDURE: Day #1 1) Collect sixteen 13X125 mm test tubes. Make sure they are clean and dry. 2) Use a sharpie and number your test tubes. Do not write on the white painted label of the test tube! 3) RT will have 4 burettes that contain sodium hydroxide and 4 that contain copper (II) sulfate. Take you set of test tubes and fill them according to the chart below. This chart will be Data Table #2 since you are collecting the absorbances on it. Make sure you follow the instructions given in the pre-lab discussion about the use of burettes. Test Tube # ml of NaOH ml of NiCl 2 Absorbance

3 Blank Distilled water 4) You will notice burettes located on lab tables #3 & 4 and #7 & 8. The burettes on table #3 & 4 contain the NiCl 2 solution and the burettes on table #7 & 8 contain the sodium hydroxide solution. 5) Go to both tables and measure out your assigned amount of the two chemicals. Make sure you record the starting and final reading from the burette. Also remember to record the molar concentration of the NiCl 2 solution and the sodium hydroxide solution. The concentration will be listed on the supply bottle at the appropriate table. 6) After you have both solutions in the test tubes, stopper each of them and then mix them thoroughly. 7) Set your test tubes in a 600 to 1000 ml beaker. Make sure that you wrote your name on the outside of the beaker with a sharpie. 8) Set the beaker and your test tubes on the side board of the lab so that they are out of the way. We will observe them some more tomorrow. Day #2 9) Retrieve your test tubes. DO NOT DISTURB THEM! We want a clear liquid with which to work! 10) Obtain 17 cuvettes. In the next step, you are going to number them. Make sure you write on the frosted side of the cuvettes. 11) Number your cuvettes 1 16 with a sharpie. Put a B on the last cuvette for the blank. 12) Using a different transfer pipette for each test tube, transfer solution form test tube #1 to cuvette #1. Fill the cuvette ~ 2/3 full. You didn t get any solid in the cuvette did you? 13) Repeat step #12 for the remainder of the test tubes and cuvettes. 14) Take your turn at the spectrophotometer, and read the absorbances for each of your cuvettes. Record the absorbances in your data table. You will be taking the reading for the wavelength that shows the greatest absorbance. Make sure you record that wavelength also. 15) When you have completed reading all your absorbances, you are ready to clean up your lab area. 16) Empty all the cuvettes that contain a colored solution into the waist container. The uncolored solutions from the cuvettes can be emptied into the sink. Rinse all the cuvettes at least 3 times 57

4 with distilled water. Use alcohol to remove the numbers from the cuvettes. Invert them onto a paper towel to dry. 17) Empty all of your test tubes into the waist container. Scrub them with soap and water, rinse with tap water and finally with distilled water. Invert them onto a paper towel to dry. 18) Make sure your work area has been tidied up. QUESTIONS AND CALCULATIONS: 7) Pick one of your test tubes #2 16. For that sample, calculate: A) Moles of NiCl 2. B) Moles of sodium hydroxide. C) Mole ratio. 8) Use your data to generate a graph. The graph will be the absorbance of the solution vs. ml of sodium hydroxide. Trim it to go into your lab book. 9) Your graphed data should indicate two lines. Draw both lines of best fit and extend them until they intersect. 10) Draw a box around the point of intersection of your two lines of best fit. Drop a perpendicular line from that box to the X-axis on your graph. What is the ml of NaOH at that point? 11) For the point you found in question #10 determine: A) the ml of NaOH B) moles of NaOH C) ml of NiCl 2 D) moles of NiCl 2 E) mole ratio of 12) Look back at question #3. How does that ratio compare to the ratio you got in #11E? 13) Copy the ratio form 11E again. (RT knows it sounds like busy work, but there will be a point!) Now, lightly cross out the chloride and the sodium ion in the units of that ratio. So what should be the ratio of the hydroxide ion to the nickel (II) ion? 14) Now consider your answer to question #13. If you assume that the charge on the hydroxide ion is 1-, what must be the charge on the copper (II) ion? 15) Does your experimental formula for nickel (II) hydroxide agree with the formula for that compound you would write from its name? Stoichiometry part of the lab: 16) Consider the solutions that gave you the negative slope on the graph. A) What color was the superannuate liquid? B) Which ion, nickel (II) or the hydroxide ion was in excess? 17) Consider the solutions that generated the line with a 0 slope on the graph. A) What color was the superannuate liquid? B) Which ion, nickel (II) or the hydroxide ion was in excess? 58

5 18) Look back at the balanced equation for this reaction. A) What is the mole ratio of sodium hydroxide to nickel (II) chloride? (This will be called the theoretical mole ratio.) B) What was the mole ratio of sodium hydroxide to nickel (II) chloride for the samples that gave you the negative slope on your graph? (These ratios will be called the actual mole ratios.) C) What was the mole ratio of sodium hydroxide to nickel (II) chloride for the samples that gave you the 0 slope on your graph? (These ratios will be called the actual mole ratios.) D) Write a sentence or two describing how you would use the information in 18 A 18 C to help you predict which reactant would limit the reaction. Source: Laboratory Investigations: AP Chemistry, by Hostage & Fossett, 2006Peoples Education, Inc This is a picture of a spectrophotometer the RT used in college! They are still good instruments; show but precise! 59

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