An apartment has a living room whose dimensions are 2.5 m 4.8 m 5.9 m. Assume that the air in the room is composed of 79% nitrogen (N2) and 21% oxygen (O2). At a temperature of 27°C and a pressure of 1.01 105 Pa, what is the mass (in grams) of the air?

Answer 1

You don't say but I think you must be quoting percent volume.
Find volume of room, I suggest, in cc and change to liters.

Change liters at conditions listed to standard conditions. Standard conditions are 101 325 Pa for pressure and 273 K for temperature. Use P1V1/T1 = P2V2/T2

corrected volume x 0.79 = volume N2 @ STP.
corrected volume x 0.21 = volume O2@ STP.

1 mol of a gas occupies 22.4 L @ STP; therefore,
volume N2/22.4 = mols N2
mols N2 x molar mass N2 = grams N2.

volume O2/22.4 = mols O2
mols O2 x molar mass O2 = grams O2.
grams N2 + grams O2 = mass of air in the room. Post your work if you need more help.