aamc 10 #39

[TripleHelix]

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I am having a really hard time visualizing this question.

Which of the following best supports the assumption that oleic acid molecules lie side by side over an acid solution but stand up over pure water?

a) The diameter of the monolayer increases in HCl. <- correct answer
b) The diameter of the monolayer decreases in HCl.
c) The volume of oleic acid decreases.
d) The volume of oleic acid increases.

My thinking is that when oleic acid is standing up it will increase the diameter over the entire length of an oleic acid molecule.
And when oleic acid is "lying side by side" the diameter should only increase over the diameter of the oleic acid molecule's cylindrical shape?

How am I supposed to be visualizing this?

 

[uoguelph]

Well if they are side by side the diameter of the monolayer would increase because the monolayer is only one molecule thick so just like you said above the diameter increases over the molecule's cylindrical shape. The other three answers don't make any sense, the volume would not decrease neither would the diameter. Hope that helps!

 

[TripleHelix]

oh so when you add the acid, the side by side molecules are adding above the monolayer that already exists?

 

[Donald Kimball]

It's the same monolayer, but the solvent changes the behavior of the oleic acid molecules. I pictured water as a pin cushion, and the oleic acid molecules as the pins. When they're stuck vertically, you can pack them tighter and occupy a small relative diameter of the cushion. In HCl, you just lay all the pins horizontally over the pin cushion in one layer. This changes the diameter from small to much larger.

 

[TripleHelix]

i think i got it.
the key point i was missing out on for the correct answer was if the same number of oleic acid molecules are used, overall pinning them vertically will increase the diameter less than laying them side by side.