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.