to solve this we need to use the ideal gas law:
[tex]PV=nRT[/tex]we need to calculate the number of mols of O2
[tex]n=\frac{m}{Mm}=\frac{4.2g}{32\text{ g/mol}}=0.131mol[/tex]Now we just have to solve for volume and use the number of mols just calculated and the standard pressure (1 atm) and temperature 273.15K:
[tex]V=\frac{nRT}{P}=\frac{0.131mol\cdot0.082\text{ }\frac{atm\cdot l}{K\cdot mol}273.15K}{1atm}=2.94l[/tex]