EXPERIMENT 9 SALTWATER CONDUCTANCE: The Effect of Concentration

Transcription

1 EXPERIMENT 9 SALTWATER CONDUCTANCE: The Effect of Concentration Introduction According to the Theory of Ionization proposed by S. Arrhenius, about 1880, ionic compounds dissolve in water forming cations and anions. Such a solution acts as an electrolyte, conducting electricity. Arrhenius determined that cations moved toward the negatively charged cathode in a battery-type set-up. Thus cations were determined to carry positive charge, and anions, which moved toward the positively charged anode, were determined to have negative charge. He found that the amount of current conducted by the solution depended on the number of ions in solution and the charges on the ions. NaCl (s) Na + (aq) + Cl (aq) Conductivity can be measured (in units of S, microsiemens) using a probe connected to a meter. In this experiment you will study the effect of increasing the concentration of an ionic compound, on the specific conductance. The concentrations of the solutions will gradually be increased by adding drops of more concentrated solutions to the beakers and the conductance will be measured. Materials needed Chemicals Figure 1 stirring rod deionized water (3) 100 ml beaker graduated cylinder 1.0 M NaCl (aq) conductivity probe 1.0 M CaCl 2 (aq) plastic dropper 1.0 M AlCl 3 (aq) Graphical Analysis Program-Excel Venier LabQuest & conductivity probe In Part 1: the student will determine which toggle setting is most appropriate for this experiment. The actual units of conductivity used are called specific conductance - the conductivity across 1 cm of solution between two electrodes in a probe measured in microsiemens or S. The toggle switch settings allow greater precision at low levels of conductance. Toggle switch settings => use this setting for conductance above about 2500 up to use this for conductance below abut use this for mid-range values of Note Baseline readings are subtracted from all readings. Different baseline readings will be shown at the different settings for example at the high setting, the baseline reading for DI water may be 100, but at the low setting maybe 10. Rinse the probe in DI water between each reading. Keep a large beaker of DI water handy. Students should work in pairs due to the number of beakers needed. In Part 2: the student will compare the conductance of three different salt solutions at similar concentration. Then in Part 3: the student will correlate concentration to conductance by doing serial dilution of one of the salts and measure the respective conductance.

2 Procedure Part 1. The purpose of this section is to determine which toggle setting is best for your experiment. You will do this by measuring the baseline with DI water and then find the maximum conductance with 12 drops of 1.0 M AlCl3 solution. The toggle setting that gives the full range is the setting to use for the rest of the experiment. Obtain a large Florence flask and fill with DI water. Clean and rinse all your beakers and flasks in deionized water. Fill a 250 or 100 ml beaker with 70.0 ml of deionized water, insert the conductance probe and record the baseline conductance for each of the toggle switch settings. Now place 12 drops of AlCl3 salt solution into the beaker of DI water, mix well, then re-measure the conductance at each toggle switch. Compare the high and low values to determine which setting would be most appropriate for this experiment. Remember, if you are using the S setting and you read a value of around 260 but on the other settings the values read in the thousands (1800), then that setting in inappropriate since it cannot measure the entire range. Procedure Part 2. Carefully clean and rinse a 100 or 150 ml beaker with deionized water. Also clean and prepare a graduated cylinder. Measure 70.0 ml of deionized water into the beaker, using a graduated cylinder. Rinse the conductivity probe with deionized water from a wash bottle. If you are using the Venier s Labquest Conductivity probe; set the selector switch on the side of the Conductivity Probe to the µs/cm range determined from Part 1. Connect the Conductivity Probe to LabQuest and then choose New from the File menu. (If you would like to collect the data on the LabQuest and have it do the graph, you must go to MODE, and choose events with entry). Submerge the probe in the beaker containing the 70.0 ml of water, as shown in Figure 1, such that the holes are completely under the water. The electrodes in the probe are on either side of these holes, so it is vital that the holes be under the level of the water. Once the number on the screen stabilizes, record the conductance ( S) on your report sheet. If Dropper Bottles of the solutions are not provided read this section, if dropper bottles are provide skip to the next paragraph. Obtain a clean medicine dropper (plastic disposable dropper) and rinse several times with deionized water. Using your 10 ml graduated cylinder, count how many drops of deionized water from the dropper is required to fill the graduated cylinder to 1.0 ml, exactly. Repeat to verify your findings. You have calibrated your dropper. For consistency, you will use this same dropper for all three solutions so remember to clean it well with deionized water between each trial. Avoid cross contamination. In your beaker containing 70.0 ml DI water squeeze one drop of the 1.0 M NaCl solution, swirl to mix well, then record the conductance. Continue this procedure, recording the conductance after each drop of 1.0 M NaCl has been added, until a total of 8 drops has been added. Be sure to rinse the probe each time with deionized water from a wash bottle. Dispose of the NaCl solution, clean both beakers and the dropper then repeat the above procedure using CaCl2, adding up to 8 drops of 1.0 M CaCl2 to fresh deionized water. Measure the conductance after each addition of a drop. Dispose of the CaCl2 solution, clean both beakers and the dropper then repeat the same experiment with AlCl3 (8 drops) and fresh deionized water. Measure the conductance. Discovery: Is the result affected by the differing sizes of the drops from the various bottles? Compare the conductance of solutions made from different bottles of the same chemical (i.e. 2 different bottles of CaCl2). Make some observations and draw some conclusions.

3 Procedure Part 3. In this section, you will learn to make a series of dilutions. Clean and re-use flasks/beakers for the following procedure. Record the conductance of the 4 solutions made below using the ( ) setting. Make sure to rinse the probe between each reading. Pour 5.0 ml of 1.0 M NaCl into a graduated cylinder then add enough DI water to have 50 ml of solution. Transfer the 50 ml of solution to a clean 100 ml beaker. Label the beaker 0.10 M NaCl. Next measure 10.0 ml of this 0.10 M solution into a clean 50 ml graduated cylinder. Use a dropper to get the meniscus precisely to the mark. Fill the cylinder to the 50.0 ml mark with DI water and pour this mixture into a new clean beaker or flask. You now have a container with 50.0 ml of M NaCl. Label the container: M NaCl. This technique is known to as SERIAL DILUTION. The mathematical equation used to determine the new concentration is M1V1 = M2V2 (the dilution equation). In this dilution, M1 is 0.10M, V1 is 10.0 ml, and V2 is 50.0 ml. M1V1 = M2 therefore (1.0 M) (5.0 ml) = M2 therefore M2 = 0.10M V2 (50.0 ml) M1V1 = M2 therefore (0.10M) (10.0 ml) = M2 therefore M2 = 0.020M V2 (50.0 ml) Next, transfer exactly 10.0 ml of this 0.020M NaCl solution into the 50 ml graduated cylinder, once again using the dropper pipet to read the meniscus precisely, fill to the 50 ml mark with DI water, and then pour this new solution into another clean container (beaker or flask). Use the dilution equation to determine the new concentration of the solution and label that container. Repeat this dilution process one more time (10 ml of X diluted to 50 ml). You should now have four distinct concentrations of NaCl solution in four different containers. Use a single 100 or 50 ml beaker and measure the conductance of each solution. Think about which order you should measure the solutions. Analysis: Calculations and Graphing Part 2: Graph the specific conductance (y axis) vs. the # of drops of solutions added (x-axis). Your instructor will advise you whether to use the Excel program or do the graph by hand. Note: If plotting this by hand, use graph paper with divisions of 5 squares/ cm or 10 squares/ inch, or smaller. Instructions for using the Excel program can be found in this manual in the Introduction. S conductance CaCl2 Determine the slope of each plot, and the equation of each line, in the form of: y = mx + b where y = the specific conductance in μs, m = the slope, x = the # of drops, and b is the y intercept (and represents the conductance of deionized water with no salt added, or the conductance of the deionized water itself). Plot all three graphs on the same paper. Take the slope of each line. # of drops Part 3: Plot the conductance of the NaCl solution (y axis) vs the concentration (M) (x axis). Determine its slope. Compare the equation from Part 2 to the equation in Part 3. How are they similar or different?

4 EXPERIMENT 9: REPORT Name: CONDUCTANCE OF SALT SOLUTIONS Section # Data: Part 1 Which setting did you choose? Part 2 Questions Drops of 1.0 M Specific Conductance ( S) solution NaCl CaCl2 AlCl Describe the appearance of each of the three graphs. Are they linear? Attach the graphs (one graph with all 3 lines on the same axes and one graph from part 3) to this report sheet. 2. Write the chemical equations for the dissociation of CaCl2 and AlCl3, similar to the one for NaCl that was given in the introduction. 3. How many ions are obtained from dissociation of: NaCl? How many ions are obtained from dissociation of: CaCl2? How many ions are obtained from dissociation of: AlCl3?

5 4. From the graphs, record the slope of each: Solution: Slope: # ions total ionic charge (sum of the absolute values of all charges) NaCl CaCl2 AlCl3 Which of the three solutions gave a graph with the greatest slope? 5. Is there any relationship between the number of ions in solution and the slope of the graph? Or does the conductance more closely approximate the ratios of the total ionic charges? Explain. Part 3. Serial dilution 1 2 Concentration of NaCl solution 0.10 M Conductance ( S) M 4 6. Discuss the similarities and differences between Part 2 and Part 3 NaCl graphs.

6 7. What is the value of b, the y intercept in the Part 2 & 3 (for the NaCl experiment)? What is the meaning of the y intercept in this experiment? (Recall: b = y value when x = 0) 8. From the graph or equation, predict what the specific conductance of a solution of AlCl3 in your experiment would be, after adding 16 drops of the 1.00 M AlCl3 to the 70.0 ml of deionized water. (You may have to use the equation of the line to calculate it, if the graph is too small to extend so far.) 9. Predict the concentration of NaCl if the conductance read 245 S. (You may have to use the equation of the line to calculate it, if the graph is too small to extend so far.) 10. If you were to compare two solutions, one of NaCl, and one of CuSO4 having the same concentration, which do you think would exhibit the greater specific conductance? Explain.