The Gas Laws

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:

constant (at constant temperature)

constant (at constant volume)

constant (at constant pressure)

Note - T stands for absolute temperature (in kelvin), not temperature in ^oC.

In fact you can combine these to produce 1 equation.

constant.

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

Use them to calculate 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 again, it will give you the same answer.

You can write this equation more usefully as:

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

Molar Gas Constant R

Using this formula can cause an obvious annoyance. Wouldn’t it be better if, instead of having to calculate a constant each time, 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

So

But what if you have half a mole? Or moles?

Simple! Use:

where n = no. of moles of gas involved.

Or write it as:

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