How can I find out which gas has the highest value of the van der Waals constant $a$?

Example set of gases in ascending order of $a$: $\ce{NO2}$, $\ce{CO2}$, $\ce{H2}$, $\ce{H2O}$ and $\ce{CCl4}$.


closed as off-topic by ron, Todd Minehardt, Geoff Hutchison, ringo, Martin - マーチン Mar 1 '16 at 4:08

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  • $\begingroup$ Taken from your comment to Max' answer, I think what you are really asking is how you can determine the value of a for any gas without looking it up. I think that would be a valid question. However, now your question was treated as if it was a homework question, because as it is posed, you simply have to look up the values. Hence it is not a good fit for our format. I recommend editing your question, so that it can be reopened and properly answered. $\endgroup$ – Martin - マーチン Mar 1 '16 at 4:13

You look up the values of "a" in a table of constants for the van der Waals equation.

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

  • $\begingroup$ Yes, but if I do not have this table which criterion should ultilizar to determine which has the greatest 'a'? type polarity? $\endgroup$ – Lucas Ferreira Loz Mar 1 '16 at 3:04
  • $\begingroup$ So I find some function of the critical temperature that gives the constant "a." How do you get the critical temperature? You look it up in a table. What is the difference?!? en.wikipedia.org/wiki/Van_der_Waals_equation#Reduced_form $\endgroup$ – MaxW Mar 1 '16 at 17:58

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