Comparing van der Waals constant for gases How can I compare the van der Waals constant $a$ (the liquefication constant) for different compounds based on my knowledge of bonding in the compound? Let's pick the following sample set:
$$
\newcommand{\d}[2]{#1.&amp;\hspace{-1em}#2}
\begin{array}{lrl}
\hline
\text{Compound} &amp; &amp;\hspace{-1em}a \\
\hline
\text{Helium} &amp; \d{0}{0346} \\
\text{Argon} &amp; \d{1}{355} \\
\text{Xenon} &amp; \d{4}{25} \\
\text{Water} &amp; \d{5}{526} \\
\text{Ethanol} &amp; \d{12}{18} \\
\text{Acetone} &amp; \d{16}{02} \\
\text{Acetic acid} &amp; \d{17}{71} \\ 
\text{Benzene} &amp; \d{18}{24} \\
\text{Toluene} &amp; \d{24}{38} \\
\hline
\end{array}
$$
A general trend that I can see is $a$ is proportional to molecular mass. Am I wrong to compare on the basis of intermolecular forces like hydrogen bonding, which is clearly very good in the water but the $a$ value isn't so promising?
Is it my misconception that $a$ depends on intermolecular forces? As far as I know, the stronger the forces, the more easily it will be liquefied, and greater should be $a.$
The reason that in your sample set water seems to have a low value of $a$ despite having hydrogen bonding is because most of the molecules that you chose were of much higher molecular mass. You should compare water with some gas of approximately equal molecular mass, then you'll see that value of $a$ in water is quite high. Your other observations are generally correct.