Aamc 9 ps # 12

[orthoboy]

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[tendiw]

Which compound in Table 1 evaporates fastest at 30 degrees celsius?

a) mtbe
b) etoh
c) etbe
d) tame

Answer is b. It says it's because it has a higher vapor pressure. Wouldn't ETOH evaporate the slowest since it can form hydrogen bonding and thus has a higher boiling point? Or is that not what it's asking?

Thanks

Yes etoh can hydrogen bond however that isn't how you should approach the question.

Vapour pressure tells you the tendency of a compound to go into the gas phase. A high vapor pressure says that the compound is more likely to be in the gas phase. Adding heat causes boiling (also now as a liquid moving into the gas phase), anything with a high vapor pressure has a tends to go into the gas phase so just a little heat is required to push it all the way into the gas phase to boil.

Vapor pressure also depends on the strength of the bonds in solution, EtOH having a high vapor pressure tells you that the hydrogen bonds aren't really strong enough to keep it in solution.

 

[whiteshadodw]

direct evidence in the passage to support something ALWAYS trumps what you think the answer should be. for that question i didn't even bother looking which one hydrogen bonded and which one didn't. EtOH evaporates the fastest at any temperature below its boiling point. at which point it technically isn't evaporating, it'll be vaporizing.

 

[UBCStudent128]

I initially chose the correct answer with the right reasoning.

THEN looked at the octane ratings and the passage specifically states that "higher the octane rating the slower it burns"
So with that logic lowest octane rating the faster it burns...

Like wtf?