How to determine which gas would have the lowest pressure?

If the volume of each sample of gas was reduced to one-tenth of its original size and the temperature remained the same, which gas would have the lower pressure. I'm getting confused because my options are $$\ce{C6H4}$$, $$\ce{C3H4}$$, $$\ce{N2}$$, $$\ce{O2}$$, and $$\ce{Cl2}$$. Our teacher told us to pick the polar compound but the problem is all of them are non polar. Would the answer be $$\ce{C3H4}$$? I have a feeling I might be drawing the lewis structure wrong because I'm positive the other four are non polar.

• Let's not forget allene as the alternative structure for $\ce{C3H4}$, and - at least in theory - $\ce{C6H4}$ could be bis-cyclopropenylidene ;) Mar 12, 2014 at 16:20

Not all of them are polar. The gases $\rm N_2$, $\rm O_2$, $\rm Cl_2$ are not polar.

The $\rm HC\equiv C-CH_3$ is polar, but $\rm H_2C=C=CH_2$ should not be and $\rm C_6H_4$ is also polar, if it is right chemical formula you wrote. But I'm not sure which is more polar, but I will say that it is $\rm C_6H_4$

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In more polar compound there will more interaction between molecules and that is why pressure will be smaller, simply because they "want" to be liquid form - not in gas.

• sorry I meant to say that they were all non polar. my bad. I'll look into the polarity of both the molecules. Thanks for your help! Mar 12, 2014 at 15:41
• I'm quite sure benzyne is not the intended compound for $\ce{C6H4}$, as it only exists briefly as a reaction intermediate. I'm don't know what other structure is more plausible, though (but as Klaus mentions, there are other possibilities, all of them rather weird). I'm half-convinced that $\ce{C6H4}$ is actually supposed to be $\ce{C6H14}$, which could be good old hexane (among other isomers). Though really, the teacher should have realized that only giving the formulae is a bad idea. Mar 12, 2014 at 16:31
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A homodiatomic gas cannot be polar, by symmetry arguments. At best it can be polarizable by dispersion forces overall. The lowest pressure would be the most strongly attracting molecules if any (e.g., water and hydrogen bonding), the most inert molecules, the smallest molecules (van der Walls equation coefficients), and then "anomalous" behaviors. Look up the coefficients (CRC Handbook)to get the real world answer.

One would guess nitrogen for its being very tightly bound and all electrons bonded. Look it up, as above.

/_\PV = energy, 101.325 J/liter-atm. Adiabatically compressing a gas tenfold will heat it. A diesel cylinder compresses 15 - 22 to one, minimum 550 C at the top of the stroke.