Dec 14, 2009
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Which gas if any, has the greater average molecular speed (equal molar quantities of two gases, O2 and H2O)?

Answer is H2O. From Graham's law, if two gases are at the same temperature, the one with the smaller molecular weight will have the greater molecular speed.

However, in my opinion, H2O will deviate most from ideal behavior. Ideal behavior for gas molecules assumes that there will be no intermolecular interactions. H2O is polar, and O2 is non polar. H2O undergoes hydrogen bonding while O2 does not. so H2O has a stronger intermolecular interactions, which will cause it to deviate more from ideal behavior, so does Graham's law.

So I think the answer is wrong, but on the other hand, maybe two substances are in gas form, so only intermolecular attraction that can be consider is dispersion force, dispersion force is increase as molecular weigh increase, so H2O has less molecular force, so move faster?

what do you think? am I right for understanding for those concepts?
 
Last edited:
Mar 16, 2010
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Philadelphia
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Hey,
I'm pretty sure that the answer is H2O b/c compared to O2, it has a smaller molecular weight, therefore it has the greater average speed (Graham's Law) And the law applies in this question b/c both of the molecules are in the gas phase. And in the gas phase, according to the kinetic molecular theory, gas molecules exhibit no intermolecular attractions or repulsion. It will be the one with the lowest molecular weight b/c it will be the "lighest", therefore move the fastest.
 

AmpedUp

The Legend Still Lives
7+ Year Member
Dec 1, 2009
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it makes sense

the lightest molecule will diffuse/effuse quicker

i wouldnt make it more complicated than that..