• I. Simulations 1. Go to https://phet.colorado.edu/en/simulation/legacy/sugar-and-salt-solutions. You can either run the (slow) web version or download the Java version. Click-and-drag the conductivity meter so that both electrodes are in the water, and click “Show values” in the “Concentration” window. Click and shake the salt shaker until its concentration is about 0.25 mol/L, and note the brightness of the lightbulb. Next, continue adding salt until its concentration is about 0.50 mol/L, and note how the lightbulb’s brightness changes. The higher the conductivity, the brighter the lightbulb. ► Answer in your notebook: Is salt an electrolyte or nonelectrolyte? How does concentration affect the conductivity? 2. Click “Remove salt,” and change “Solute” to “Sugar.” Add sugar to the water until its concentration is about 0.25 mol/L, and note the brightness of the lightbulb. ► Answer in your notebook: Is sugar an electrolyte or nonelectrolyte? 3. Click the “Micro” tab at the top, use the salt shaker to add some sodium chloride crystals to the water, and observe what happens when they dissolve. Click “Remove solute,” change “Solute” to “Sucrose,” use the shaker to add some sucrose crystals to the water, and observe what happens when they dissolve. (You can try different solutes by clicking the right arrow in the “Solute” window if desired.) ► Answer in your notebook: Which crystal breaks up into ions in water? Which crystal breaks up into molecules in water? How does this explain your results from Parts 1 and 2? 4. Go to https://phet.colorado.edu/sims/html/acid-base-solutions/latest/acid-basesolutions_en.html, and click “Introduction.” Click on the lightbulb in the lower-right corner, and drag the electrodes into the solution. Note that the lightbulb lights up very faintly; this tells us that pure water is actually a very weak electrolyte, but we will assume in our experiment that it is essentially a nonelectrolyte. Observe how the lightbulb’s brightness (and thus the conductivity) changes when you try solutions of a strong acid, weak acid, strong base, and weak base (on the right). By looking through the magnifying glass, you can correlate the conductivity to the amount of acid or base that has ionized. ► Answer in your notebook: Classify each of the solutions (strong acid, weak acid, strong base, and weak base) as a strong electrolyte, weak electrolyte, or nonelectrolyte. 5. View the following animation to see a molecular-level view of a precipitation reaction: https://preparatorychemistry.com/precipitation_Canvas.html. II. Calibrating the Conductivity Probe 1. Watch the prelab video at https://www.youtube.com/watch?v=sWPrhQKR39Y. 2. On the computer desktop, the “Gen_Chem” folder is opened and the file “Conductivity” is chosen. Live conductivity readings should appear in the upper and lower left corners of the screen. The range switch on the probe adapter box is set to “0-20000 μS/cm.” 3. From the “Experiment” menu, “Calibrate” is selected and “Calibrate Now” is chosen. The conductivity probe is immersed in deionized water, and “0” is entered in the left calibration box, then “Keep” is chosen. The probe is dabbed dry and immersed in the calibration standard (NaCl, 1000 μS/cm). The conductivity value of the standard (written on the bottle) is entered in the right calibration box, then “Keep” is chosen, followed by “Done.” The conductivity probe is now calibrated. The probe must be rinsed with deionized water before attempting to take any other readings, and also when switching between solutions. Once the probe is calibrated, it does not need to be calibrated again during the lab period. III. Conductivity of Electrolytes vs. Nonelectrolytes 1. Watch the video at https://mediaspace.minnstate.edu/media/1_iazt1osv. 2. 100 mL of deionized water is placed in a 150-mL beaker, and a magnetic stir bar is added. The beaker is placed on a stir plate, and a clamp is used to hold the conductivity probe in the liquid. Record the conductivity reading for pure deionized water. (The value will fluctuate, so use either the average or “most common” value.) ► For the hand-in: What does this reading tell you about the behavior of water as an electrolyte or nonelectrolyte? 3. Approximately 0.2 g of sugar is weighed out (record the mass to the thousandths place) and added to the beaker while stirring. When all of the sugar has dissolved, record the conductivity reading. ► For the hand-in: What does this reading tell you about the behavior of sugar when it dissolves in water? 4. The sugar solution is discarded, and 100 mL of fresh deionized water is added to the beaker. Approximately 0.1 g of sodium chloride is weighed out (record the mass) and added to the beaker while stirring. Record the conductivity reading. ► For the hand-in: What does this reading tell you about the behavior of sodium chloride when it dissolves in water? 5. An additional 0.1 g of NaCl is weighed out (record the mass) and added to the solution from Step 4. Record the conductivity reading. ► For the hand-in: How much did the reading change? What does this tell you about the relationship between conductivity and concentration? 6. The salt solution is discarded, and 100 mL of fresh deionized water is added to the beaker. About 60 mL of the stock sulfuric acid solution is obtained in a small, clean, dry beaker. Most of this will be used in the next section. For this part of the experiment, only 3 drops of the stock sulfuric acid solution is added to the beaker of deionized water while stirring. Record the conductivity reading. Next, 3 more drops are added. Record the new conductivity reading. ► For the hand-in: What do these readings tell you about the behavior of sulfuric acid in water? IV. Determination of the Concentration of a Barium Hydroxide Solution 1. Watch Dr. Edvenson perform the experiment at https://mediaspace.minnstate.edu/media/1_i4ox4o7u. 2. Record the concentration of the H2SO4 solution in your lab notebook. A buret is rinsed thoroughly with 2-3 mL of stock sulfuric acid solution. This rinse is discarded into a waste beaker, and the rinsing is repeated an additional one or two times. The buret is then filled with stock sulfuric acid solution. The buret is placed in a buret clamp on a ring stand. The buret clamp is positioned near the top of the ring stand to provide a more stable mount for the buret and to allow additional clamps to be placed lower on the stand. 3. A little over 25 mL of the barium hydroxide solution is obtained from the stock bottle. A graduated cylinder is used to measure out 25.0 mL of the Ba(OH)2 solution into a clean 150- mL beaker with a magnetic stir bar. Record the volume of Ba(OH)2 that is measured out to the tenths place. The beaker is placed on a stir plate, and a clamp is used to hold the conductivity probe in the solution. If the liquid in the beaker is not deep enough to immerse the bottom portion of the probe, additional deionized water is added to the beaker. ► Question for thought: How does adding water at this point affect the measurements/results of the experiment? 4. In Logger Pro, the “Collect” button is clicked. This should activate the “Keep” button immediately to the right of “Collect.” With the conductivity probe in the solution, “Keep” is clicked. A dialog box pops up, “0.00” is typed for the total volume of acid added at this point in the experiment, and “OK” is clicked. ► For the hand-in: What does this reading tell you about the behavior of barium hydroxide in water? 5. The initial buret reading is recorded (to the hundredths place), and approximately 1 mL of sulfuric acid is added from the buret to the barium hydroxide solution in the beaker. The exact volume of acid added is recorded (to the hundredths place). You do NOT need to record the acid volumes or conductivity readings during the titration in your notebook. The solution is allowed to stir for a few seconds until the conductivity reading stabilizes, then 4 “Keep” is clicked, and the total volume of acid added to the beaker is entered. Observe and record any changes in the appearance of the solution. 6. The addition of sulfuric acid in approximately 1-mL increments is continued (always recording the exact amount used), clicking “Keep” and entering the total volume of acid added since the beginning of the experiment. At least 6-8 data points are collected after the conductivity reaches a minimum value, at which point “Stop” is clicked. 7. Two trends are observed on the graph (posted as a separate file on D2L), and “Analyze: Linear Fit” is used to add a trendline to each trend. Record the equations for the trendlines in your lab notebook. If you can’t read the values on the screen, they are (with units omitted for clarity): Decreasing trend: 4 1 y x =− + × 577.5 1.230 10 Increasing trend: 4 2 y x = −× 738.3 1.572 10 where y is the conductivity value and x is the total volume of H2SO4 added. 8. The equivalence point of the titration occurs where the two lines meet, which means their y and x values are equal. Thus, set the two trendlines equal to each other ( 1 2 y y = ) and solve for x, which will be the volume of H2SO4 solution (in mL) required to reach the equivalence point. You can report this value to the hundredths place, corresponding to the precision of the buret. 9. Use the volume of H2SO4 required to reach the equivalence point (just calculated, but converted to L) and the known molarity of the H2SO4 solution to compute the number of moles of H2SO4 required to reach the equivalence point. Write the balanced chemical equation for this reaction, and use the mole ratio to determine the number of moles of Ba(OH)2 in the solution of unknown concentration. Divide this by the volume of Ba(OH)2 solution used (in L) to determine the molarity of the Ba(OH)2 solution. Show your calculations for Steps 8-9 in your lab notebook. Be sure to round your final answer to the appropriate number of significant figures. Questions for Analysis The following questions are designed to help you interpret your data. You are not required to record the answers to these questions in your lab notebook. 1. Why does the conductivity change as it does in the graph? How is this change in conductivity related to your observations of the solution’s appearance? 2. What is the significance of the point of minimum conductivity? What is the relationship between the number of moles of Ba(OH)2 originally present in the solution and the number of moles of H2SO4 added to reach this point? Using Conductivity to Explore a Chemical Reaction Hand-In, Chem 150L, Fall 2020 Due Friday, Oct. 30, 11:59 pm Name: Partner: 1. (2 points) Based on Parts I and III of the experiment, explain why a sugar solution does not conduct electricity while a sodium chloride solution does conduct electricity. By comparing your salt solutions, what did you observe regarding the relationship between the conductivity and concentration of a salt solution? {Type answer here.} 2. (3 points) Write a correctly balanced complete chemical equation AND net ionic equation for the reaction of sulfuric acid with barium hydroxide. You must include the phases of each reactant and product and appropriate charges in the net ionic equation. You must also format the formulas correctly using subscripts and superscripts. No excuses for sloppy equations. {Type answer here.} 3. (9 points) Be sure to answer each part of each of the following questions, which relate to the titration of barium hydroxide with sulfuric acid (Part IV of the experiment). a. What substance is causing the initial solution in the beaker to conduct electricity before any reactant from the buret is added? Is this substance a strong electrolyte, a weak electrolyte, or a nonelectrolyte? If appropriate, identify the ions that are responsible for the conductivity of the initial solution in your beaker. {Type answer here.} b. As sulfuric acid is added from the buret, what substances are increasing or decreasing in the system as the conductivity is decreasing, and why does this cause the conductivity to change in the way it does? Connect these ideas with what you observed happening in the beaker and with what you know about whether these substances are strong, weak, or nonelectrolytes. {Type answer here.} c. In what way does the conductivity trend change over the course of the experiment? What substances are increasing or decreasing in the system after the trend changes, and why does this cause the conductivity to change in the way it does? Again, discuss this in terms of what type of electrolyte(s) is/are present and, if appropriate, identify the ions responsible for the observed conductivity. Finally, in connection with this, what is the significance of the point of minimum conductivity? {Type answer here.} 4. (5 points) Based upon your titration results, calculate the concentration of the original stock barium hydroxide solution. Also include the graph for the titration (copy the picture from D2L). Show all the steps of your calculations using Equation Editor and briefly explain what you are doing in each step in accompanying text. {Attach graph here. Be sure it is sized large enough to read easily.} {Type calculations and explanations of calculations here.} 5. (1 point) Since we did only one run of this experiment, it is more difficult to deal with error in a statistical manner. What are some potential sources of error in this experiment? Recall that “human error” is a mistake, not a legitimate source of error. Do not list “human error,” such as “parallax error.” {Type answer here.} See the attached rubric for more detailed information about grading. Q#1 Explain conductivity Q#2 Eq’n. Unsatisfactory Both explanations are missing or unclear. Borderline Either explanation is missing or unclear. 0 points No equations given. 1 point 1.5 points -0.5 pt for minor formatting errors. 1 point Substance or ions not properly identified. 2 points -0.5 pt for minor formatting errors. 0 points Q3b None of the three As H2SO4 items addressed is added correctly. 1 point Two of three items not addressed correctly. 2 points One of three items not addressed correctly. 0 points None of the three items addressed correctly. 1 point Two of three items not 2 points One of three items not addressed correctly. Q3a Before rxn. starts Q3c 0 points Nothing properly identified. Satisfactory Excellent Explanation of reason for conductivity and of the relationship between concentration and conductivity is clear. 2 points Correct molecular equation, 1 pt. Correct phases, 1 pt. Correct net ionic equation, 1 pt. 3 points Substance, 1 pt. Nature of substance, 1 pt. Ions identified, 1 pt. 3 points Substances increasing/decreasing properly identified and related to conductivity and observations. 3 points Conductivity change, chemical substance responsible, and Pts. 2 pts 3 pts 3 pts 3 pts 3 pts After trend changes addressed correctly. 0 points Q#4 Conc. of Ba(OH)2 Q#5 Disc. of error Total No calculations and no graph. 0 points Unclear or poor discussion. 0 points 2 points 1 point Maximum score possible if Equation Editor is not used for calculations. 1 point -0.5 pt for minor errors. 3-4 points significance of point of minimum conductivity properly identified. 3 points Graph included, 1 pt. Calculations are correct and include units, 2 pts. Sig. figs., 1 pt. Clear explanation of calculations, 1 pt. 5 points Reasonable discussion of sources of error. 1 point 5 pts 1 pt 20pts
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