Determining the Conductivity of Standard Solutions

Transcription

1 Determining the Conductivity of Standard Solutions by Anna Cole and Shannon Clement Louisiana Curriculum Framework Content Strand: Science as Inquiry, Physical Science Grade Level Objectives: 1. The learner will use proper technique to make a standard solution of NaCl. 2. The learner will perform a series of dilutions and determine the correct concentration of each solution. 3. The learner will determine the conductivity of each solution using a Computer Based Laboratory and a Conductivity probe. 4. The learner will create a graph exhibiting the linear relationship that exists between the concentration and conductivity of the diluted solutions. 5. The learner will determine the concentration of an unknown by measuring its conductivity and interpolating the graph to determine its concentration. Teacher Information Benchmarks SI-H-A1, SI-H-A2, SI-H-A3, SI-H-A7, SI-H- B2, SI-H-B3, SI-H-B4, PS-H-A1, PS-H-A2, PS-H-C3 Time Frame 1 class period (60 minutes) Curriculum Integration Science: Properties of Solutions, Technology, Conductance of solutions, taking quantitative measurements, concentration of solutions, lab safety Math: Graphing the results of an experiment, determining the best-fitting straight line of a series of points (Linear Regression), using a Graphing calculator to graph and interpret results Materials ml volumetric flask ml volumetric flasks TI-83 Plus Graphing Calculator CBL, conductivity probe, DIN adapter, NaCl, 150 ml beaker, Electronic or Triple Beam Balance, pipet, pipet bulb, 7 test tubes, DI water. 10 ml graduated cylinder Language: Writing a lab report using correct format and grammar.

2 Applications Laboratory technician (chemical plants, hospitals), environmental researcher, chemical engineering, fishing industry Student Groupings Students working in pairs Possible Obstacles to Student Learning 1. Operating the CBL and TI-83 Plus graphing calculator. 2. Calculating the diluted concentrations of the solutions. 3. Using quantitative techniques when preparing solutions. Opportunities for Assessment 1. Formal Assessment of Lab Report. 2. Questions on lab activity will be graded to check for understanding. 3. Group work will be monitored and checked for lab safety, proper techniques, cooperation, and participation as the lab is being completed.

3 Lesson Procedure Prelab Assignment: Complete on a separate sheet of paper to be turned in before class. Record a second copy for your records to use with lab. 1. Determine the amount of salt needed to make 250 ml of a 0.5 M NaCl solution. 2. Determine the diluted concentration as being described below using VcCc =VdCd a. 2 ml of a 0.5 M NaCl solution is diluted to a final volume of 100 ml of solution. b. 4 ml of a 0.5 M NaCl solution is diluted to a final volume of 100 ml of solution. c. 6 ml of a 0.5 M NaCl solution is diluted to a final volume of 100 ml of solution. d. 8 ml of a 0.5 M NaCl solution is diluted to a final volume of 100 ml of solution. e. 10 ml of a 0.5 M NaCl solution is diluted to a final volume of 100 ml of solution. Making a Standard Solution 1. Make 250 ml of a 0.5 M NaCl solution. a. Determine the mass of NaCl needed to make the standard solution. Show all calculations on lab report. b. Place a 150 ml beaker on an electronic balance and press the TARE button to zero the balance. c. Add the appropriate amount of salt to the beaker. d. Dissolve the salt in a small amount of water (about 100 ml). e. Quantitatively transfer the solution into a 250-mL volumetric flask. f. Rinse the beaker with small amounts of water to get all of the salt solution into the flask. Repeat the rinsing at least 3 times to assure that all salt is transferred to the volumetric flask. (Be sure not to exceed 250 ml when rinsing the beaker.) g. Dilute solution to the 250 ml mark. Dilution of the Standard Solution 2. You will now use a series of dilutions to get varying concentrations of sodium chloride to be tested with the conductivity probe. 3. First Dilution: Pipet 2 ml of the 0.5 M solution and add to a 100 ml volumetric flask. Dilute to the mark. 4. Second Dilution: Pipet 4 ml of the 0.5 M solution. Add to a 100 ml volumetric flask. Dilute to the mark. 5. Third Dilution: Pipet 6 ml of the 0.5 M solution. Add to a 100 ml volumetric flask and dilute to the mark. 6. Fourth Dilution: Pipet 8 ml of the 0.5 M solution. Add to a 100 ml volumetric flask and dilute to the mark. 7. Fifth Dilution: Pipet 10 ml of the 0.5 M solution. Add to a 100 ml volumetric flask and dilute to the mark.

4 8. Prepare a data table to be placed in your lab report which includes the following: sample description, calculation of dilution using the dilution formula, and final concentration. 9. Label 7 test tubes 0,1,2,3,4,5 and U (unknown) and place in a test tube rack. 10. Place 10 ml of de-ionized water in test tube Place 10 ml of the first dilution in test tube 1, place 10 ml of the second dilution in test tube 2, etc. until all test tubes have 10 ml of diluted solution. Measuring the Conductivity of the Diluted Solutions 12. Once the solutions are prepared, set up a conductivity probe into the DIN adapter and plug it into Channel One on the CBL (always use the lowest possible channels). Connect the CBL system to the GC with the link cable using the port on the bottom edge of each unit. Firmly press the cable ends. 13. On the conductivity probe, set the conductivity value to microsiemens. 14. Be sure the CBL and calculator are turned on. 15. Press APPS key, select Chembio and then press enter until you get the main menu. 16. Select set up probes and press enter. Enter 1 as the number of probes being used in the experiment. Select conductivity from the select probe menu. Enter 1 as the channel number. Select Used stored from the calibration menu. 17. Select microsiemens from the conductivity menu 18. Select collect data from the main menu. 19. Select trigger/prompt from the data collection menu. 20. Lower the probe until the hole of the probe end is completely submerged into the water. 21. Monitor the conductivity of distilled water displayed on the CBL screen. (Units are microsiemens, µs) 22. Once the reading has stabilized, press trigger on the CBL, and then enter 0 as your concentration. The conductivity and concentration values have now been saved for the first trial. This gives the conductivity of de-ionized water. 23. Select more data. 24. Now submerge the conductivity probe into the first diluted solution. 25. Once the reading has stabilized, press trigger and then enter the concentration of the first dilution as calculated from the prelab assignment. The second set of data has now been stored. 26. Rinse the conductivity probe with de-ionized water. Repeat the procedure for each of the remaining solutions. Remember to rinse the probe with de-ionized water between each sample. 27. Once all the test tubes have been read, press STOP from the data collection menu. Your points will be displayed but not yet connected. Press. When prompted to repeat press NO. 28. Press View Data and follow the instructions on the screen. Record all data from L1 and L2 into Lab report, labeling each set of data with an accurate description. Be sure to record your data, it will be deleted when the unknown is analyzed. 29. Return to the ChemBio program by pressing APPS. 30. To connect the points to form a straight line, select fit curve. Then select LINEAR L1, L2,,, then select SCALE TO DATA. A straight line will be formed as a result of your dilutions. Use the arrow keys move from one data point to another data point. This straight line will allow you to determine the concentration of an unknown NaCl solution. (By knowing the conductivity, you will be able to interpolate your graph to determine the concentration of the unknown solution.)

5 Determining the Conductivity of the Unknown Solution 31. Now use the calculator to determine the conductivity of your unknown solution. a. Press to clear your graph. b. Rinse conductivity probe with de-ionized water. c. Obtain an unknown from the instructor and record the unknown number in your lab report. 32. Press COLLECT DATA 33. This time select SINGLE POINT. 34. Place conductivity probe into your unknown solution and follow the instructions on the screen. When the reading is complete, record it into your lab report. Press. When prompted to repeat press no. 35. Press 7 to quit the ChemBio Program so that you can manually input your data points. 36. To do this press and edit or. Clear any unwanted data in each list that was entered as a result of analyzing your unknown. Highlight the list (L1), then press and. Reenter the concentration in L1 and the conductivity in L2 (if necessary). 37. You must now form a linear regression (best fitting straight line) using your data points. a. Press to view the settings. Plot 1 should be turned on. Press to view the settings on plot 1. On should be highlighted on Plot 1, scatter plot should be highlighted., Xlist: L1 and Ylist:L2. Then press (QUIT). b. Press. Arrow over to (CALC) and press 4:LinReg (ax+b) to plot your data points or coordinates as a function of the y variable. Press L1 L2. Arrow over to Y- VARS and press. c. Press and 9 to view graph. (if necessary) d. You are now ready to determine the concentration of the unknown using your straight line and your conductivity results. 38. Press. On Y2 = enter your conductivity reading. Then press (Calc). Press 5:Intersect and press. Read the x value at the bottom of the screen. Record concentration from graphing calculator on lab report. 39. Print this screen using Graph Link. a. Once the program is opened, select Link from the program s top menu bar. b. Choose Get Screen from this window. Your calculator screen will appear in the window box. c. Enlarge screen to at least 200%. d. Choose Print then OK in the printer dialog box. e. Choose Done and exit the graph link program. f. Label the graph properly and turn in with lab report. 40. Print graph using the graphing analysis program. a. Attach the Graph Link cable to the calculator. b. Start the Graphical Analysis for Windows program. c. From the Menu bar select FILE and IMPORT FROM TI CALCULATOR. d. On the calculator press 2 nd LINK. Select 4:LIST. Press. Select L1 and L2, using the arrow keys to highlight the list; then press to select it. Arrow over to TRANSMIT. Press ENTER. The list should appear in the data columns on the computer. If not then check the Com Port by using the up and down arrows on the computer and selecting ComPort 1.

6 e. Rename the data lists by moving the cursor onto the Data Table window and double click on L1. In the column options dialog box, type in an appropriate new name for the column of data and new units describing the data. Click OK and repeat this step for L2. f. Click anywhere on the Graph window. Click on GRAPH on the top menu bar. Choose Connecting Lines to remove the line graph drawn through the data. g. Click on Analyze on the top menu bar and choose Automatic Curve Fit. In the Automatic Curve Fitting dialog box, under Stock Functions, choose the Linear function then click OK. h. Click on OK-Keep Fit (on the right hand side near the bottom of the screen.) i. Type a description of the graph in the Text window along with your group s name. j. Double click on the floating box which gives the slope and y-intercept. Un-check Box is visible and press OK. k. Select FILE and PRINT from the menu bar. 41. Turn this graph in with your lab report. Attachments Attachment 1. Rubric Attachment 2. Sample Data that should be include with each lab report Attachment 3. Questions (Key) Exploration and Extension Questions: 1. Determine the independent and dependent variables in this experiment. Provide an explanation of your choice. 2. Are the reactions that occur in this experiment chemical or physical? Explain. 3. Provide a general statement concerning how conductivity relates to the concentration of a solution. 4. Write the dissociation reactions for NaCl, CaCl 2, and AlCl 3 in water. 5. Predict which solution would have the highest conductivity reading and explain your answer. 6. Provide a general statement concerning how conductivity relates to the number of ions in a solution.

7 Attachment 1. Rubric General Observations of lab activity Always Sometimes Not observed observed observed Lab safety Participation Proper use of equipment Subtotal = Lab Procedure Excellent Good Average Poor Prelab answers Lab report format Spelling and Grammar Data tables Excellent Good Average Poor Units Graph (Graph Link- labeled) Graph from Graphical Analysis Lab report turned in on time Calculations Significant Digits Identification of the Unknown Analysis Subtotal = Answers question Answers question Does not answer no answer Correctly with clear correct; no examples question correctly hand-writing Understanding using to support answer limited understanding cannot be Examples to support adequate under- no evidence to read answers standing of question support answer Question Question Question Question Question Question Subtotal = Your evaluation Lab partners evaluation

8 *the sum of both evaluations must equal to 10 when added together Total possible points A Subtotal = Your score: B Your grade: C D

9 Attachment 2. Sample Data Data Table 1-1 Concentration and Conductivity of Diluted Solutions Sample De-ionized water Dilution 1 Calculations (Using VcCc=VdCd) (M) Final diluted Concentration (M) Dilution 2 Dilution 3 Dilution 4 Dilution 5 Table 1-2 Data stored in L1 and L2 of the Graphing Calculator Concentration in Molarity (L1) Conductance in Microsiemens (L2)

10 Table 1-3 Conductivity and Concentration of Unknown Solution Unknown Number Conductance (µs) Concentratio n (M) (graphing calculator)

11 Attachment 3. Questions (Key) 1. Determine the independent and dependent variables in this experiment. Provide an explanation of your choice. The independent variable is the variable that we change in the experiment. In this experiment the concentration was changed, and we want to see how the change in conductivity depends on the change in concentration. The dependent variable is the response to the change in the independent variable (the concentration). In this case the change in conductivity is the outcome that is produced by changing the concentration. 2. Are the reactions that occur in this experiment chemical or physical? Explain. The dissolving (dissociation) of salt is a physical change. The salt loses none of its chemical properties as a result of placing it in water. The salt can easily be reformed by evaporation or distillation of the water. 3. Provide a general statement concerning how conductivity relates to the concentration of a solution. The greater the concentration of the solution, the greater the conductivity of the solution. 4. Write the dissociation reactions for NaCl, CaCl 2, and AlCl 3 in water. NaCl Na +1 + Cl -1 CaCl 2 Ca Cl -1 AlCl 3 Al Cl If NaCl, CaCl 2, and AlCl 3 had the same concentration, predict which solution would have the highest conductivity reading and explain your answer. When the concentration is the same (same number of formula units) the conductivity can still be effected by other factors such as number of particles or ions that are produced when the salt dissociates. Conductivity readings are directly related to the number of ions that exist in solution. The higher the number of ions that are present the higher the conductivity reading will be. When NaCl dissociates in water it breaks up into 2 ions as is indicated by the following equation: NaCl (s) Na +1 (aq) + Cl -1 (aq). In CaCl 2 solution, because the stoichiometric relationship is 1:2, three ions will be formed by the dissociation of one formula unit of CaCl 2 as is indicated by the following equation: CaCl 2 (s) Ca +2 (aq) + 2 Cl -1 (aq). Likewise when AlCl 3 dissociates in water four ions will be formed according to the following equation: AlCl 3 (s) Al +3 (aq) + 3 Cl -1 (aq). Therefore, mole for mole, aluminum chloride will produce more ions and will yield a higher conductivity reading. 6. Provide a general statement concerning how conductivity relates to the number of ions in a solution.

12 The greater the number of ions that are produced when a salt dissolves, the greater the conductivity of the solution.