Lab moves quickly today, with the last student leaving about 45 minutes before the end of the period.
The manual says that TAs will assign dyes this week - you may do this, or let your students choose their own as long as they choose something w/ λmax >50nm away from their previous dye. Some of them don't understand that the 'shoulder' on a peak is really a second, overlapping peak. I had some questions about why the dyes in the mixture have different absorbances even though their concentrations are the same (it's because of ε). Have them take a complete spectrum of their new dye or look at the spectrum of a group from last week before predicting the combined spectrum. Also remind them again that they need to take a reference/blank with DI water.
The unknowns from the stockroom are PAAS easter egg dye tablets or kool-aid powder. Have your students use a SMALL amount of solid unknown...they should break or scrape pieces off of the tablet rather than dissolving the entire thing. Even then, they usually end up with too high of a concentration. Most students' instinct is to take all 200mL of concentrated dye and add another 400mL of water to dilute instead of taking a 1mL aliquot and adding 2mL water to that.
Make sure your students record their procedure for both Parts B and C. This is a good lab to differentiate between quantitative and qualitative techniques...here they want to identify the dye, but they don't need to quantify anything. They could measure the mass of dye used and calculate a concentration in units of g/mL, but it is not necessary for identification. It's up to you how detailed you want their procedure, so make sure you specify if you will take pts off for missing masses.
This is the fun part. Each student should do their OWN unknown. M&Ms and skittles work great if they use about 5 pcs of the same color and leave them in 5 or 10mL of water for only a few seconds (don't dissolve the white part or the chocolate). The stockroom will provide hotplates (if you ask) for dissolving solids, or concentrating dilute liquid samples. Leave the troubleshooting to the students rather than giving them a quick answer. They have plenty of time to think for themselves this week. If they brought an unknown that won't work (cranberry juice, brown soda etc), have them try it anyway and then explain to you why it didn't work. Also, stress that there is no food or drink consumption allowed in lab!
Typically one student uses the computer and the other student mixes chemicals. Since this experiment is so close to what they did last week, I use this lab as an opportunity to have partners switch roles. They don't like it at first, but it seems to help their comfort level and confidence in future experiments.