The Prelab: A Mechanism for Student Feedback

101_2493Grading lab reports this week I was disconcerted that so many of the students had problems with the calculations required of the lab (EDTA Titration of Zinc Chloride). The lab manual gave a detailed example of the calculations and the prelab was almost entirely composed of step-by-step calculations that were completely analogous to those required by the actual lab. In my mind, the real focus of the lab report should be interpreting the student’s data and comparing with the class – not trying to figure out how to do the math required by the experiment. This semester, I am checking each student’s prelab at the beginning of lab and having them hand it in with the lab report to be graded the following week. Checking the prelab at the beginning of lab allows me to check up on each student with a moment of personal interaction which is a good thing. On the other hand, they are getting the lab set up and the prelab check is more-or-less of a distraction. Another possibility would be to go over the prelab in detail at the beginning of the lab period. This is tempting, but not a very efficient use of time – the right lesson but the wrong time. In Organic Chemistry, I would have them hand in the prelab before the lab so I could grade it and return it to them before the lab report was due.

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One Comment

  1. Posted October 8, 2009 at 8:13 pm | Permalink

    I totally agree–labs should be about interpreting data more than actually trying to figure out what the data is–the latter we do in class. But I think there is a deeper issue than students just not being able to ‘follow and repeat’ the experiment example in the pre-lab. Chemistry is different than math in that there are different approaches to understanding logic. Most of the people I met/still meet who struggle in chemistry don’t struggle in doing the math–they find it hard to connect the logic used in chemistry to what exactly they need to add/subtract/multiply etc.

    For example, I know someone who knows how to write molecular formulas of compounds and balance equations. But she/he doesn’t understand how certain compounds behave with each other–like, carbon and hydrogen compounds always turn into H20 and CO2 when burned. So maybe people know how to calculate moles and grams, but maybe they can’t make the link between Zinc and EDTA. I don’t know–you graded the lab reports and you know better.=) But the people I met totally didn’t get the relationship of EDTA to Zn or how if you have too much Zn, you’ll get too little Cl and that tripped them up when trying to calculate the empirical formula. So it wasn’t an issue of ‘math logic’ but an issue of ‘chemistry logic.’ (Again, I could be COMPLETELY wrong–I just happened to get questions from at least six people for that lab and that is what I noticed….)


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