In general you are more likely to receive an answer if you describe the problem you are facing and include any error messages. In this case 'Undefined function or variable 'Pv'.', caused by
Pv=[Pv P'];
To fix it you need to declare Pv before using it.
Tc=369.9;
Pc=42.0;
Omega=0.152;
%%
%universal gas constant
R=83.14;
% b and m for the PR EOS
b=0.7780*R*Tc/Pc;
m=0.37464+1.54226*Omega-0.26992*Omega^2;
j=1;
Pv=[];
for i=40:10:90
%molar volume
v=0.001:1:2500;
%temperature
T(i)=273.15+i;
%reduced temperature
Tre=T(i)/Tc;
%a for the PR EOS
a = 0.45724*(R*Tc)^2/Pc*(1+m*(1-sqrt(Tre)))^2;
%PR EOS
P=R*T(i)./(v-b)-a./(v.*(v+b)+b*(v-b));
%plotting isotherms for T varying from 313.15 to 363.15
h=plot(v,P);
set(h,'color',rand(1,3),'linewidth',2);
hold on
axis ([0 1600 -40 60])
xlabel('volume in cm3/mol')
ylabel('pressure in bar')
title ('isotherm for propane')
end
