calculate the mass of 500 ml of O₂ at 27⁰C $ 740 mm of Hg

Since value of R is in L atm we will convert the volume in L and pressure in atm
V = 500 ml = 500/1000 = 0.5 L
P = 740 mm Hg = 740/760 = 0.97 atm
R =0.082057 L atm K ^-1 mol -1
T = 27⁰C = 27 +273 = 300 K
Putting the values in formula: PV = nRT
n = PV /RT = 0.97 atm * 0.5 L / 0.082 LatmK^-1 mol^-1 * 300 K = 0.485 / 24.6 = 0.02 moles
Number of moles = given mass / molecular mass
given mass = number of moles * molecular mass = 0.02 * 32 = 0.64 gram of O₂
​Mass of O₂ = 0.64 g