Hi

why do we need to write mol % when measuring oxygen purity in air? why not just say % since mol is a counting unit?

Thank you

 

Because you want to count molecules, not atoms. Although air will have the same percentage either way.

 

Oxygen purity?

Well I would wish to know more about the detail.

How are you defining oxygen purity?

Air contains other components than just gas.
Particulates, smoke, water droplets, variable water vapour etc

Are you referring to dry air as a basis?

 

In general when you give a fraction of something, you need to say what that is a fraction of.

For instance, if you say "mass-%", you are saying that the total mass of the oxygen is that fraction of the total mass of the air. If you say "volume-%", then you are saying that the total volume of the oxygen is that fraction of the total volume. Saying "mol-%" means that you are saying that the total number of molecules of oxygen is that fraction of the total number of molecules in the air. In general these are all different, although for an ideal gas the volume-% and the mol-% would be the same.