The easy way to answer this is to just look up the density of air $\rho$ under whatever conditions you need it and then calculate $V=m/\rho$.
If you don't have a ready source for the density you need, though, you can use the ideal gas law,
$$PV=nRT$$
Here $n$ is the number of moles of the gas, which you can get by dividing the mass by the average molar mass of the gas:
$$n=\frac{m}{\mu}$$
The average molar mass of air would be a weighted average of the molar masses of its constituent gases. This is something you look up for the composition of air under your specified conditions, but it'll be around 29 g/mol. This at least does not depend directly on temperature.
If you need more accuracy, you could use something like the Van der Waals equation.
From your comments it seems that effectively you are asking about "why do gases mix so easily?"
If a system such as a mixture of gases is kept under constant temperature in a constant volume, the equilibrium state corresponds to the minimum of Helmholtz free energy:
$$A = U - TS$$
As you see, for $A$ to reach the minimum either the energy $U$ should decrease or the entropy $S$ should increase (or both in reality).
Minimizing energy. Most of the energy of common gases at normal conditions comes from their kinetic energy defined by the temperature. Energy due to intermolecular potential is negligible. So the only possibility to lower the energy is to lower the gravitational energy. In essence it would require the mixture to perfectly separate --- heavy gases at the bottom, light gases up.
Maximizing entropy Maximum entropy for the system in hand (under specified conditions) would imply perfect mixture, the state of most disorder. That's actually what drives the diffusion.
So as you see, the equilibrium state is a compromise between low energy and high entropy. For gases the entropy wins, because there isn't much energy difference between a mixture and a separated state (apart from gravity, which is still small).
As for your example with oil and water the situation is opposite. Unlike gases considerable amount of energy in liquids comes from intermolecular forces. Thus there is a huge differences in energy of interaction water-water or water-oil, so it is more preferable do separate to considerably minimize the energy.
Best Answer
Air can be considered to be an ideal gas. This means that the volume percents of the given gases are also the mole percents of those same gasses. Accordingly, you need to find the weight fraction of each of the given gases. This is done by multiplying the mole fraction by the molar mass for each gas to arrive at a pseudo-mass. Do this for all gases and add up all of the pseudo-masses to get a total pseudo-mass. Divide all of the pseudo-masses by the total pseudo-mass to get mass fractions.
For example, for a case where there is 0.21 mole fraction of oxygen and 0.79 mole fraction of nitrogen, the oxygen pseudo-mass is 0.21*32 (oxygen is diatomic with a molar mass of 32), which equals 6.72. The nitrogen pseudo-mass is 0.79 * 28 (nitrogen is diatomic with a molar mass of 28), which equals 22.12. The total pseudo-mass is 28.84, which is the average molar mass of air with the stated composition. Divide the individual pseudo-masses by 28.84 to arrive at 0.233 weight fraction oxygen and 0.767 weight fraction nitrogen.