January 2011 Archives

Experiment 1 Part 3 - Titrations using Conductivity

Lab today was easier than last week since the students are more familiar with the equipment and the layout of the lab. Most of my students were finished in about two hours, and one student stayed until the end.

Check-out and assembly are the same as last week. Remind the students to plug their computers into the power or the batteries will die during lab. They will also need to log in to the network to print their graphs.

Pre-Lab Discussion:
I discussed the definition of a precipitation reaction and drew out the complete and net ionic equations from their prelab questions. Many of them did not know how to write ionic equations, and this helped them think about the concentration of ions in solution and conductivity. From there, I discussed the uses of titrations, the equivalence point and how to properly set up and read a buret - keep buret vertical, use a funnel to fill, fill the tip before beginning, and keep the buret tip and cup close together to eliminate splashing. I also mentioned that burets can be read to two decimal places, and they measure the amount dispensed rather than contained, but I still had about half the groups write "1,2,3,4..." and read the buret incorrectly. It helps to double-check a measurement for each group just to make sure they are reading it properly.

Other tips:
Students can stir their reaction with the probe rather than using a stirring rod. There is no need to rinse the probe between measurements here.

Ask your students why their solution is turning cloudy. Most of them will be confused and scared at first, but eventually they figure out that it's due to the solid AgCl that is forming. They should be recording their observations after every addition.

Computer use was much more fluid this week. Several students tried to add a linear or curved trendline, which is unnecessary for this experiment.

Almost every group asked me about Q16 - how to calculate the original AgNO3 concentration. It helped to draw out a beaker with ions floating in it, write the chemical equation and talk about moles of ions and converting from moles of one reactant to moles of the other reactant. For some reason, many of them wanted to use the 0.028g of NaCl and the molar mass to convert to moles of AgNO3 instead of the volume at the low point on their graph.

All silver-containing waste goes in the "Expt.1 Waste" jug at the end of the benches.

Exp 1 Parts 1&2 Chemical Conductivity

These comments were already emailed, but here they are again for future reference:

The lab today took the entire class period. Remind your students that the computers MUST be turned in 15 minutes before the end of lab.

Check-Out: In previous semesters, the students have needed to check out a 10mL graduated cylinder. This year they have those in their drawers, so they do not need them from the stockroom.

Assembly: Make sure that your students plug everything in BEFORE turning on the computer, including the conductivity probe and the power cord. I had a couple of groups lose their excel graphs when their batteries died. Have them use Appendix F to set up their probe. The directions are fairly clear, but a common problem is that the students open the incorrect file (see the file path in Appendix F). Make sure the silver lever on the probe is set to 0-20000uS.

PART I: The stockroom provides plastic conductivity cups that can be reused for each measurement and then disposed of after the experiment. It saves time to have your students fill their plastic washbottles with DI water and wash using these instead of walking back and forth after every sample. They should also rinse their probe and dry with a kimwipe between samples. Also remind them that the probe window must be completely submerged to get an accurate reading, which may require tipping the cup. I showed them how to use tthe autodispensers during my prelab because I have seen students fiddling with the knobs and forcing the flow rate. You may want to check your students' conductivity values: DI water should be low, between 1 and 50, tap water closer to 200 and ionic solutions in the thousands.

PART II: Remind your students how to use analytical balances, even if you told them last week. Make sure that they clean up after themselves. Some TAs find it useful to designate a different pair of students "Lab Monitors" for each experiment to help keep the benchtops neat. Show your students how to use their volumetric flasks. Some of my students tried to measure their 0.028g NaCl directly into the flask. Some of my students struggled with the dilution calculations. Refer them to the M1V1=M2V2 examples in Appendix D. It helps if they write out the units for every number in the equation. Have them record the sample number for their unknown solution. The unknowns can be disposed of when they are finished.

Graphing: I emailed my students before lab and had them email me the excel exercise from Appendix B before class, which lessened the number of questions I received. Make sure they use an xy-scatter plot, and show them how to add a trendline and R2. Remind them that their graph should include a title (including their and their partners' names), and axis labels with units.

Printing: Students must be signed onto the network before they can connect to the printers. To do this, they can open internet explorer and sign in with their username and password. They can then print to the red or green universal printer. Make sure that they have highlighted the graph ONLY so they do not print pages of numbers.

Waste: Acids and bases go in one small beaker. Everything else goes down the drain. There is no baking soda set out for neutralizing, but all of my students' waste was close enough to neutral that it could go down the drain with a bit of water.

Watch for goggles. They like to wear them around their neck or on top of their head. If you catch them now, it's easier later in the semester. I also had several students chewing gum or drinking water in lab.

Test 1

test entry

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