The thing to remember about STP conditions, i.e. Standard Temperature and Pressure, is that the volume occupied by exactly
SIDE NOTE That value corresponds to the old definition of STP conditions, i.e. a pressure of
In the current definition, the temperature is
Sadly enough, not that many textbooks and online resources have adapted to the change, so the old value is still going strong. I'll use the old value here, but keep in mind that the correct value is not
The volume occupied by exactly
In your case, you know that
#11.2 color(red)(cancel(color(black)("L O"_2))) * overbrace("1 mole O"_2/(22.4color(red)(cancel(color(black)("L O"_2)))))^(color(blue)("molar volume of a gas at STP")) = "0.500 moles O"_2#
To convert the number of moles to grams, use the molar mass of oxygen gas -- do not forget that oxygen gas exists as a diatomic molecule!
#0.500 color(red)(cancel(color(black)("moles O"_2))) * "32.0 g"/(1color(red)(cancel(color(black)("mole O"_2)))) = color(darkgreen)(ul(color(black)("16.0 g")))#
The answer is rounded to three sig figs.