Even more musing on the mathematics of the Ideal Gas Law

I have noticed that a common mistake when doing computations involving the Ideal Gas Law is failing to use the correct units. You need the correct units, if you are going to make the mathematics work properly.

The calculations require that the variables be absolute mathematics to work.

Well, for volume (V) that is easy, there is not really a negative volume. So there is an absolute volume rather than relative volume.

And, this holds true for the amount of substance (n). In this real world, you got stuff or you got nothing.

Temperature (T)on the other hand can be measured relative or absolute.

Looking at WIKI

http://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Celsiushttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Conversion_of_units_of_temperatureAbsolute zero

Kelvin 0.00

Celsius -273.15

Fahrenheit -459.67

Rankine 0.00

Delisle 559.73

Newton -90.14

Réaumur -218.52

Rømer -135.90

Again we need to select the correct units to do the calculations. That may mean you have to convert from the temperature units you have to the absolute temperature units. Many people favor Kelvin over Rankine.

I have seen for pressure (P) measurements the old tire gauge is used. Well, most everyday gages start off at zero so that means they started with atmospheric pressure built in to the measurement. So PSIG (pounds per square inch gauge) is the same as approximately 14.7 PSIA (pounds per square inch absolute). In many of the question you get on test, the questions are biased by giving you the absolute pressure. But, sometimes the testers trick you. In any case, if you do not have absolute pressure, convert what you have to absolute.

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Now we have everything converted to an absolute units and then we need to select constant (R) to go with those units.

WIKI has a whole list of potential values of the constant.

http://en.wikipedia.org/wiki/Gas_constantFor instance if our units are Litre Atmosphere Kelvin Mole

0.08205746 L atm K−1 mol−1 would be used for the constant (R).