answer and explanation
we are given the volume of the gas at STP as 30 L
we know that the gas equation is
PV = nRT
we can rearrange to make n subject of the formula like :
n = PV/RT
at STP
p = 1 atm
T = 273K
R = 8.314
therefore
n = (1 atm)(30 L)/ (8.314)(273 K)
= 0.0132 mols
now that we have the mols we can calculate the mass using the equation
the molar mass of O2 is 32 g/mol
m = nM
= (0.0132 mols)(32 g/mol)
= 0.42 grams