The gas laws

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The gas laws

We need to know more about how gases behave if we are to make this really useful. The problem is that real gases are confusing in the way they behave so just to make things easier for ourselves, scientists consider a pretend, ideal gas which behaves perfectly.

The three gas laws this ideal gas obeys perfectly are:

pV = constant (at constant temperature)

Copyright S-cool constant (at constant volume)

Copyright S-coolconstant (at constant pressure)

Note: T stands for absolute temperature (in kelvin), not temperature in °C.

In fact you can combine these to produce 1 equation.

Copyright S-coolconstant

In other words, find a value of p, V and T.

Use them to calculate Copyright S-cool and no matter what you do to the gas (as long as you don't add or lose some of the atoms) whenever you calculate Copyright S-cool again, it will give you the same answer.

You can write this equation more usefully as:

Copyright S-cool

which shows that the value for Copyright S-cool at moment 1 is the same as the value of Copyright S-cool at a second moment 2.

No doubt, you've spotted an obvious annoyance. Wouldn't it be better if, instead of having to calculate a constant each time you did an experiment you had a single constant that was always the same?

Of course - but you would always have to have the same number of atoms/molecules in each sample for this to be the case.

So someone said - what about if the number of atoms/molecules was Avogadro's number? i.e. you had 1 mole of gas?

Well, if you calculate the value of the constant for that you always get the same number, 8.31JK-1mol-1. So it's called the universal molar gas constant symbol R.

Universal molar gas constant, R = 8.31 JK-1mol-1


Copyright S-cool

But what if you have half a mole? Or Copyright S-cool moles?

Simple! Use:

Copyright S-coolwhere n = no. of moles of gas involved.

Or write it as:

pV = nRT

This equation is called the Equation of State for an Ideal Gas or the Ideal Gas Equation.