Let's look at the relationship between concentration and pressure of a gas.

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The ideal gas equation is: 

where p is the pressure, V the volume, n the number of moles of gas, T is the absolute temperature and R is the gas constant. Pressure can be measured in atmospheres or Pascals (1 atm = 101,325 Pa).

We can rearrange the ideal gas equation as follows:

If p is measured in atmospheres, V in dm^–3 and T in kelvin, what are the units of the gas constant R?

atm dm^–3 K^–1 mol^–1

If p is measured in Pa, V in m³ and T in kelvin, what are the units of R?

Pa m^–3 mol^–1 K^–1

Because Pa m³ = J, how might the units of R be written?

J mol^–1 K^–1

The equation can be rearranged as follows:

But n/V is the concentration of the gas in mol dm^–3.

So we can write  where [gas] is the concentration of the gas.

So at constant temperature the pressure of a gas is proportional to its concentration: