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