How many molecules are there in 4.00 l of oxygen gas at 500.∘ c and 50.0 torr?

From  the  ideal  gas law   
pv=nRT , n  is  therefore PV/RT
R is  the
R is  gas  constant =62.364 torr/mol/k
P=500torr
 V=4.00l
T=500+273=773k
n={(500 torr x 4.00l)/(62.364 x773k)}=0.041moles
the  number  of  molecules=moles  x avorgadro costant that is  6.022x10^23)
6.022 x 10^23)  x0.041=2.469 x10^22molecules

Yes you are correct use the scientific method