Ideal gas equation PV=RT is not followed by real gases. Real gases follow Van-der-waal equations in which pressure is substituted as $P+\frac{a{n}^2}{V^2}$ and volume term is substituted by ($V-nb$ ). i.e.,  $(P+\frac{a{n}^2}{V^2})(V-nb)=nRT$ where ‘a’ and ‘b’ are called Van-der-waal constant. By the help of ‘a’ and ‘b’ we can study critical phenomenon of gases.