AAMC test 10 PS #41

[kabtq9s]

I ended up randomly guessing on this question cause I ran out of time. But if I did have time I would have picked that it wouldn't work because water would disperse over the oil, thinking of balls of water forming on top of oil. The answer was that it wouldn't work because H2O is not cylindrical. Why is C wrong though? And how do we know that water molecules aren't cylindrical ?


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[MT Headed]

The question stem asserts that the water molecules will form a monolayer, so your concern about water beading is unfounded.

Answer C is incorrect because the water molecules engage in hydrogen bonding with each other and will not disperse across the surface.

We know water individual molecules aren't cylindrical because they are bent (V-shaped). They don't have one dimension that is significantly longer than the other two.

Answer D is correct and answers A and B are wrong, because the technique of experiments 1 and 2 uses the areas of the oil spots to estimate the height and the diameter of the cylindrical molecules, and therefore the molecular volume, and therefore avogadro's number.

I suspect that the experimenter could just assume that water molecules have a spherical shape (you know, like in physics, where you assume a spherical horse of radius 'R' ) and run only experiment 1. The experimenter would then have a reasonable estimate for the radius of a water molecule, which leads to the volume of a water molecule, which leads to avogadro's number. But that is not consistent with the question stem.

 

[kabtq9s]

Thanks again MT Headed. These types of passages/ questions make me