October 2010 Archives

Notes on Experiment 16:

- Go over the details of the well prep carefully. This includes not cutting to the bottom of the dish, not putting wells too close to each other, etc. Basically, make sure they know to take all precautions for keeping the solutions inside the wells and not all over the place.

- Mention that the red lead is, contrary to most of the electronics world, the ground. They should be using the black and green leads to take measurements.

- Use Raw Voltage 0-5 (not any of the other options) when you set up the voltmeter.

- Impress on students the importance of washing their hands if they don't want to be eating lead or cadmium with their lunch/dinner.

- I suggest to my students that they bend the metal strips so that one end is in the well, and the other is resting on the edge of the dish. This allows them to simply touch the leads to the metal strips, rather than painstakingly clipping and unclipping each one and/or removing and replacing strips from the wells.

- From Lindsay: having your students start the agar gel prep before prelab can also save time; this allows you to give your prelab lecture while the gels are cooling.

--They will have one colored (metal) solution next week. These are generally easy to identify, but we've had a problem in the past with KI being yellow. It is yellow again this week. If it is still yellow next week, the students should be able to identify the difference between KI and the chromium solution, which is also yellow.

--Remind the students that some of these solutions are quite strong (i.e., 6M HCl) and to keep their hands clean

--Students can cut down on their time substantially if they avoid solutions with common ions and make sure to follow their solubility rules (both on page 15-2).

Also, please make sure that the balance areas are kept clean. Although the balances themselves were clean when Stephen and I arrived this morning, the benchtops around them are crusted with acid and salt solutions that have dried out since last week. Thanks!

Notes for this week:

1. Students should keep their NaOH and unknown acids for next week. Do not let them neutralize the leftover materials.

2. Impress on students the importance of not touching solid NaOH with their hands. If they do, they should wash their hands immediately.

3. Show the students what "bad" and "good" endpoint looks like when a student gets one.

4. Keep the balances clean of KHP and NaOH - these are particularly corrosive, and they will get dirty.

--The manual asks for them to wait ten minutes for their solution to settle in the wash. This is not necessary. As soon as they are done with the calibration, they can move on. The rinses only need a few minutes sitting if they have been agitated.

--During the setup of their six wells, some of them may get a brown or black solution instead of the appropriate orange solution. Have them add additional KI.

For Waste Management:

--Have your students use two filters. It substantially speeds up the process of filtering off the Sr(IO3)2.

--If your students have a black/purple precipitate, have them add more ascorbic acid. With enough swirling, the ascorbic acid will yield a clear solution.

For this week's experiment, Stephen and I have only a few things to contribute:

1) The molar absorptivity value for this lab is 4296. Write this down for your students. (Also, b = 1 cm).

2) Make sure their spectrometers are set to 447 nm before they start taking data. It's easiest if you set it to Abs v. time, because they then can visually see the equilibrium.

3) for Part 2, tell them to only report the colors that they actually see. Also, somehow none of them ever seem to notice that the net ionic equations it asks for are on the preceding page.

That's about all we have for this week. Good luck!