Gas Pressure and Weight for Dummies

Gas molecules under pressure

Unlike so many of the content farmers who spread copious amounts of freelance manure across the internet a few years ago, here at the Antisocial Network most of our staffers have training in the STEM fields (Science, Technology, Engineering and Mathematics). It makes no difference whether it's formal education or on-the-job skills, they've acquired at least some scientific literacy. That's apparently not the case of many eHow.com contributors who "claimed titles" in science, though: instead, they relied on their crack journalism skills... as did historian cum sociologist cum professional journalist William "W. D." Adkins the day he tried to show readers "How to Figure the Weight of Gas in a Cylinder"¹ at Sciencing.com.

Adkins apparently thought this would be a simple geometry problem with a little bit of physics thrown in to make it more complex. That's more or less what he said in his introduction:

"Air, as well as gases such as hydrogen and helium, has mass. If you could put a gas on a scale, you’d find it has a specific weight that depends on the density of the particular kind of gas. However, you can figure the weight of gas in a cylinder if you calculate the volume of the cylinder and know the density of the gas it contains."

Our staffer who found that statement immediately blurted, "Wait: you need to know more!" but a quick scan of William's text showed that he didn't know that. Here's what W. D. told the OQ to do: Find the radius of the cylinder... Measure the height of the cylinder... ...calculate the volume [of the cylinder using] the formula V = H x pi x R^2 Find the density of the gas in a table of gas densities. Multiply the volume by the density to figure the weight of gas in the cylinder.

And, according to Adkins, you're done. Which, as anyone who has ever picked up a 20-lb propane cylinder could tell you, is utter bull. You see, Adkins' alleged solution does absolutely nothing about the pressure of the gas in the cylinder! No, even a high-school chemistry student knows about his one, but neither Adkins nor the (supposedly) expert content editor thought of it...

The real answer? You need to calculate the number of moles of gas in a pressurized cylinder using the Ideal Gas Law, PV = nRT. You know pressure (P)... W. D. showed you how to calculate volume in liters (V), the universal gas constant (R) and the temperature (T) -- then solve the equation for n, the number of moles of gas. Then figure the mass at approximately 28 grams per mole of gas. In other words, a cylinder at 1000 psi contains a greater mass of gas than a cylinder at 14.7 psi.
Adkins obviously didn't know jack about this. The content editor obviously didn't know enough to correct him. And so, we have yet another case where we truly wish the Antisocial Network could hand out two Dumbass of the Day awards, but we can't – so W. D. gets another one all to himself (number three, for those who are counting).

¹ The original has been sent to the cleanup team by Leaf Group, but can still be accessed using the Wayback machine at archive.org. Its URL was   ehow.com/how_5976192_figure-weight-gas-cylinder.html

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