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.

About this Entry

This page contains a single entry by amaxwell published on January 31, 2011 4:55 PM.

Exp 1 Parts 1&2 Chemical Conductivity was the previous entry in this blog.

Experiment 2 Part 1 - Chemical Reactions is the next entry in this blog.

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