If you have n_{i} moles of each species (for
example n_{A} moles of A), you can try to calculate
the pressure or volume that that gas alone (i.e., ignoring
the other gases that are around) exerts/occupies.

**Partial pressure** refers to the pressure that
would be exerted by a species (in a mixture) if there were
no other species present.

The pure component volume, v_{A}, refers to the
volume that would be occupied by a species (in a mixture)
if there were no other species present.

So, in an ideal gas mixture EACH COMPONENT satisfies the
ideal gas law provided the partial pressure or pure component
volumes are used!

P_{A}V = n_{A}RT

or

Pv_{A} = n_{A}RT

In this way, the sum of the component pressures (partial
pressures) or volumes (pure component volumes) should sum to
the total pressure or volume:

P_{A} + P_{B} + ... = P

V_{A} + V_{A} + ... = V

This is easy to see if you divide either the partial
pressure equation or the pure component volume equation by
the ideal gas law for the total mixture:

$\frac{p_AV = n_ART}{PV = nRT}$

$\frac{Pv_A = n_ART}{PV = nRT}$

Note that RT cancels in both equations and that V cancels
in the first and P cancels in the second, also that
*n _{A}/n* =

p

v

So The volume fraction (or pressure fraction) of an ideal
gas is equal to the mol fraction! (v_{A}/V =
n_{A}/n)

Determine the composition of a mixture of ideal gases from their partial pressures or volume fractions