The mole fraction is expressed as parts per million (ppm).

The sum of all the mole fractions is equal to 1:

The zero air is put directly into the cylinder if very low final mole fractions are targeted.

These measurements are usually made as mole fractions, for reasons explained here.

It makes up 0.187% of natural calcium by mole fraction.

Graphs of H vs mole fraction can be found in the literature for many acids.

In the same way, increasing the mole fraction after a pressure drop brings the relative humidity back up to its initial level.

Why do we express the abundance of CO2 as a mole fraction in dry air?

In reality the mole fraction of the target gas is very accurately known.

The target gas has low mole fraction in this case, registering at 1.1 volt.