Wednesday, 23 January 2013

Ladies and gentlemen: commit!

Last week, my 1st Year Chemistry lab was tasked to carry out a redox titration.  What we used to call ferrous ammonium sulphate when I last did such an experiment, is now called Iron II ammonium sulphate.  The first step was to make a 0.1M solution of (NH4)2.Fe.(SO4)2.6H20.  This is a solution that contains 6.02 x 10^22 (1/10th of Avogadro's number) molecules of the salt in every litre.  To achieve this you have to calculate what the molecular weight of the compound is, then weigh out  1/10th (0.1M) that number of grams, add it all to 1 litre of water and stir.  You can find the mol.wt. of each of the relevant atoms (H=1, N=14, O=16, S=32, Fe=56) from a periodic table and the molecular formula tells you how many of each species there are. So the computation should be completely straight forward – why a child of six could do it!  But the night before class, it took me three goes to get it right.  You never really learn unless you try and get it wrong a tuthree times, so I was happy enough.
One problem is that nobody adds (can add?) in their head anymore - because we have calculators on our phones.  In my first science-qualified job in 1983, I was in charge of massive 1st Year Genetics classes which generated large amounts of individual data.  These had to be aggregated to get a class total.  One of my graduate teaching assistants used to stand in front of the blackboard where the results were tallied, adopt a fixed expression, make a faint whirring sound and after a couple of minutes announce the answer.  Where are you now when we need you, Rob?
So in class last week, I wrote up the formula asked the class to calculate the molecular weight and write it down in top right hand corner of their manuals in ink and put a box round it.  5/15 got it right, which was about as good as I did.  And nobody was allowed to pretend that they knew how to do it really, and would have got the right answer if they’d had time to check it.  So I think that was a valuable lesson.  But how do you know what the right answer is, if there is no neighbour to copy from or teacher doesn’t tell you? One method is to add the numbers up in two different ways:
By groups (the natural way). (NH4)2. = 36 + Fe = 56 + (SO4)2 = 192 + 6H20 = 108
By atoms (20xH=20; 2xN=28, 14xO=224, 2xS=64, 1xFe=56)
If they tally, you just might have the right answer.  Being confident enough to stand over your calculations/results/data is an essential part of the training to become a scientist.

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