Using van der Waal's equation for 1 mol:
(P+aV2m)(Vm−b)=RT
Since the volume is negligible, neglecting b in the above equation we get:
(P+aV2m)Vm=RT
Given : T=80.0821K
PVm+aVm=R×80.0821
PV2m−8Vm+a=0
According to the question, Vm=12 L=0.5 L so,
P(0.5)2−8(0.5)+2=0
P=8 atm