how many molecules of air are there in 1 litre flask at 293K, if the preessure in it is 0.3 atm.
Use the ideal gas law:
P*V = n*R*T
Solve for n, the number of moles
R = 0.08206 l-atm/mole*K
T = 293
n = 0.01248 moles
Multiply that by Avogadro's number for the number of molecules.