Ranking gases by value of the van der Waals equation-of-state constant 'a'?
You look up the values of "a" in a table of constants for the van der Waals equation. 

\begin{array}{lrr}
   \text{gas}                  &amp;
    a / \mathrm{L^2bar\,mol^{-2}} &amp;
    b / \mathrm{L\,mol^{-1}}      \\\hline
   \text{Hydrogen}             &amp;    0.2476  &amp;         0.02661 \\
   \text{Carbon dioxide}       &amp;    3.640  &amp;         0.04267 \\
   \text{Nitrogen dioxide}     &amp;    5.354   &amp;         0.04424 \\
   \text{Water}             &amp;    5.536  &amp;         0.03049 \\
   \text{Carbon tetrachloride} &amp;  19.7483  &amp;         0.1281  \\
\end{array}