The concentration of a compound in a particular phase can be represented with a wide spectrum of units. Typically, a mass per volume (kg of compound per m3 of the phase) structure is readily understood. However, many other forms find there use and there way into practice. Other forms include mass per mass (kg/kg, wt%, wppm, wppb etc.), mole based versions (mol/mol, mol%), volume per volume (m3/m3, vol%, vppm etc.), and partial pressure (Pa). Not to mention the non SI system units.
Converting from one set of units to another is an essential skill. This conversion process frequently relies on a compound’s molar mass and the phase’s density. The ideal gas law proves quite useful as a source of density in gas phase situations (provided pressures are moderate to low and temperatures are not really low).