Density for the Dummy Chemistry Student (Metric System 3)

Density in action...

One of our staff recalls, sometime in the '80s, seeing an order form for cabinets that warned the buyer to "Give units in inches because metrics is unamerican." As the French are wont to say, "Plus ça change, plus c'est la meme chose": the more things change, the more things stay the same. You do know it's an election year, right? Whether "unamerican" or not, the metric system is here to stay and if we patriotic Americans want to communicate with the rest of the world, we frequently need to convert units. Why anyone would need a "Gallons to Kilograms Conversion" isn't particularly clear (probably middle-school science), but Brenda Scottsdale of eHow.com (now Sciencing.com) tackled this one in typical freelance style.

As everyone knows by now, Demand Media (which has since changed its name to Leaf Group, presumably to protect the guilty) demanded that Brenda begin with a 75-100 word introduction. Well, things start out more or less OK in her intro, but then quickly go downhill. Here: see what we mean:

"Gallons are an English measure of volume while kilograms are a Metric unit of mass. Therefore, you need to know the density of the liquid you are measuring to convert gallons to kilograms. Weigh the substance for which you have the volume measurement, then convert this weight to kilograms."

Umm, yeah, sure. She has all the right words: volume, mass, density -- though we aren't very sure what that instruction has to do with everything... For what it's worth, Brenda, a gallon is specifically a unit of liquid volume: you should have mentioned that. Whatever the case, that's about the only thing Scottsdale got right, because soon she misinforms her readers that

"Mass is a measure of the density of a substance. The weight of the water that fits into a glass, for example, is a measure of mass."

No, Brenda, mass isn't a "measure of the density of a substance"! in fact, density, if used correctly, is a measure of mass per unit volume! Based on that moronic misstatement, however, Brenda goes on to provide "instructions": "Measure the density of the substance you wish convert by weighing it." "[Convert] to kilograms..." Weigh the substance and then convert the weight to kilograms? That's all you have to do? We call bullshit, Brenda: First off, you don't have to weigh the "substance" at all; you just use the liquid's density to calculate. Here, try this:

1. Obtain the liquid's density in pounds/cubic foot (example: gasoline, 45.4 lb / ft³)
2. Multiply by 0.1337, the number of cubic feet in a (US, as opposed to Imperial) gallon (equals 6.07 lbs/gallon)
3. Divide by 2.2, the number of pounds in a kilogram: the answer is 2.76 kg

Brenda, unfortunately, thinks kilograms are metric units of density and as a result just plain gets her "answer" wrong. And here you were wondering how we could ever think a "six sigma black belt" could be the Antisocial Network's Dumbass of the Day. Need we say more?

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