Lab took almost the entire period today with the first students finishing in 2 hours and the last about 5-10 minutes before the end of class. They should be working in partners.
Pick up your TA evaluation forms from Smith 115. These go straight to you so that you can improve your teaching. I get better feedback if I ask them to write at least one good thing and one thing that needs improvement when I hand them out.
Show the students how to make an ice bath to rinse their precipitate with cold DI water. They should start with this so that their water is cold by the time they need it. The lab manual does not mention an ice bath, so they will skip this step if you don't tell them to do it. Also ask them why they should use ice cold instead of boiling water because none of my students understood this.
Many students didn't know that [ ] means concentration, so they were confused by the table on p 3-4. Many only recorded one sig fig from the bottle instead of all three (0.3M instead of 0.300M)
It can save time to have each group measure 50-51 mL of each reactant initially - enough for all three trials and a few drops leftover for Part 3. If they decide not to do this, make sure that they label their grad cylinders with the reactant they contain because if they get switched, they will end up with a reaction occurring in their grad cylinder.
Remind your students to weigh their filter paper for each trial. As soon as they have their first trial filtering, they can move onto their second as long as they label everything. There are sticky labels under the chalkboards. After filtering, they can unfold their filter paper and place it on watch glasses or the cover of their spot plate to dry over the weekend.
MAKE SURE THEY DON'T DISPOSE OF THEIR FILTRATE BEFORE COMPLETING PART 3.
I discussed limiting and excess reactants during my prelab by drawing a beaker containing the reactants with an arrow going to a beaker containing the products. I proposed three different molar feed ratios and asked my students which of the reactants would still be present at the end. They seem to understand Q3-9 better when you use "excess" reagent rather than "limiting" reagent when you lead them to the answer.
I also drew a diagram of the spot plate wells on the board:
T1 T2 T3
Ca(NO3)2 O O O -> Q3-6
NaCO3 O O O -> Q7-9
Before they leave, they should complete a separate page in their notebook with their % yield calculations set up for next week. Exp 5 takes the entire period, so it is vital that they have this ready before they get to class next week. It should take them no more than 5 minutes to weigh their samples and turn everything in. I had them write a conclusion this week in which they discussed potential reasons for their %yield being >100% and <100% so that, whatever their results, they won't have to do this next week.
WASTE: Everything can go down the drain.