When Chemists Attack

One thing that most new science students have trouble doing is keeping proper track of units.  Beginning students will write down numbers, assured that they and everyone else will just know what units the number has.

Ten minutes later they’ve forgotten what the units of their numbers were and completely mess up their calculations.

Well, here’s yet another bad thing that can happen when you don’t pay attention to units.

OPP said the U.S. woman’s Mercedes Benz was clocked at a speed of 140 km/h heading west on Highway 401 just before 10:30 p.m. Sunday.

[…]

The total fine amounted to $405.

According to police, the Californian’s explanation for speeding was that the speedometer in her Benz only gave readings in miles per hour and she wasn’t familiar with the metric system.

She was driving in a 100 km/hr zone.  If we’re to believe this woman’s excuse, she saw the “100 km/hr” sign and thought “Whoo!  100!  That means I can drive 100 miles per hour here!  Floor it!”

(Now if she really didn’t understand the system, she might have considered using a little common sense.  Her speedometer at the time would have been reading about 87 miles per hour, which is enough to get a ticket most places in the US.)

So here we have a case of mistaken units costing a woman a lot of money.  Something to think about if you’re a new student feeling a little lazy in the lab …

There’s a simple relationship that we use quite a bit in analytical chemistry: the relationship between how concentrated a solution is and how much light it absorbs. It’s called Beer’s Law, and in simplified form, it looks something like this:

… where A = Absorbance (measured by a spectrometer), c = concentration, and k = a constant***. Even if you’re not familiar with Beer’s Law or the instrument it’s used with, you can see that this is a simple linear relationship. If you know the value of the constant k, then you can measure the absorbance of a solution on the spectrometer, then use the equation to find out how concentrated the solution is.

So how do you get the constant? Just make some solutions where you already know the concentration, and measure the absorbance of each one with a spectrometer. The raw data looks something like this.

Now, what would you do to find that constant, k, that relates the absorbance and the concentration? Here’s a hint – you’ll need to plot the data and perform a linear regression analysis to find out the value of k. This might sound hard, but a modern spreadsheet can make a nice looking plot and perform the linear regression. All you have to do is enter the data, tell the spreadsheet what things to plot, and let the spreadsheet do the grunt work.

Some of my students had to plot absorbance and concentration data as part of a recent laboratory experiment. A depressingly high percentage of these students produced this plot.

Quick! What’s wrong with this plot? (And no fair peeking below to see the answer!)

After making the plot, these students used the value for k that the spreadsheet calculated to find the concentrations of their unknown samples … and failed miserably – reporting concentrations that were several orders of magnitude too high. Impossibly high.

So where did the students go wrong? Their plots should have looked like this.

Beer’s Law, after all, is a relationship between absorbance and concentration, not between absorbance and the numbers arbitrarily assigned to each test tube for identification! To further the problem, many of these students did not even notice that the concentration numbers they reported were ridiculously wrong.

I’ve been teaching freshman chemistry full-time for more than seven years, and this sort of mistake is much more common now than it was seven years ago.

James Cameron was wrong. The machines will take over eventually, but not via squads of semi-indestructible Schwarzeneggers. They will simply rob us of our ability to think.

*** This constant depends on several things, including the identity of the substance you’re analyzing and the size of the spectrometer’s sample holder.

My introductory chemistry class is geared primarily towards students who are getting their associate’s degree in a medical field – mainly nursing. Because of this, we use drug dosage calculations for math practice when we discuss metric unit conversions.

Some students think I’m harsh when I don’t give credit for botched drug calculations. I, of course, disagree – and Abel Pharmboy’s found a news item that illustrates why:

A pharmacy erroneously made a drug 10 times more potent than intended, which killed three people who received it at an Oregon clinic, the state medical examiner said Friday.

if any of my students are reading this, take note. Stupid math mistakes are sometimes amusing, but never when someone’s life is at stake!