Lesson 2 - Gas Laws

We previously discussed the concept of moles as a measure of gas quantity, but why we would use such an abstract unit? Particularly when it is related, via molecular masses, to mass (kg) itself? Would it be easier in practice, to use mass? Yes, it would!

An alternative approach to the universal gas law (and constant) is to customise the law (and constant) for a specific gas which allows mass (m) of the specific gas to be used. In this way, the universal gas law becomes the following:

Where m is the mass of gas in kg and R_Specific is the specific gas constant with units of J/kg K. If we compare the two forms of the gas law, we can see the following:

And:

Equating them gives:

And:

Rearranging for R_Specific:

If we remember how we calculate n, the number of moles as the ratio of mass to molar mass (M. Mass):

Which simplifies to:

So, the specific gas constant R_Specific is the ratio of the universal gas constant R divided by the molecular mass of the gas. We can see how to calculate this in the following worked example.

Example:

What is the specific gas constant for carbon dioxide (CO₂) gas?

The molecular mass of CO₂ is 12 + 2 x 16 = 44 kg/kmol.

Consequently, R_Specific for CO₂ is:

The table below shows the specific gas constant for common brewery gases.

Table: Specific gas constants for common brewery gases