AAMC 10 chem q's

[chiddler]

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1. Regarding the passage with the oil drop over water and finding avogrado's number. I didn't understand how the experiment found avogrado's number just by placing a known mass of oil on water. What is the design here?

2. Number 40.

Which of the following solvents could NOT be used to dissolve the hydrocarbon portion of oleic acid?

*A. Acetic acid
B. Benzene
C. Diethyl ether
D. Chloroform

Why is chloroform a good solvent? Why is acetic acid a bad solvent? Chloroform should be quite polar!

 

[MedPR]

1. Regarding the passage with the oil drop over water and finding avogrado's number. I didn't understand how the experiment found avogrado's number just by placing a known mass of oil on water. What is the design here?

2. Number 40.

Which of the following solvents could NOT be used to dissolve the hydrocarbon portion of oleic acid?

*A. Acetic acid
B. Benzene
C. Diethyl ether
D. Chloroform

Why is chloroform a good solvent? Why is acetic acid a bad solvent? Chloroform should be quite polar!

Not sure about #1, I'd have to look at it.

#2: Chloroform may be polar, but it can't H-bond with itself.

 

[chiddler]

yeah...i suppose that's right. damn...

 

[MedPR]

yeah...i suppose that's right. damn...

POE is really helpful for that particular question. Two non-polar and two-polar, but three non-h-bonders and one h-bonder. So the lonely guy (h-bonding one) is probably the answer.

 

[Dasypus]

For #1:

The volume of the drop times its density gives you the mass of the drop. If you know the volume of the drop and the volume of each molecule, you know how many molecules are in the drop.

Avogadro's number is the number of molecules in a sample that weighs in grams what the molecule weighs in AMUs. We know what oleic acid weighs in AMUs because we know its formula. So if we've got the number of molecules in any given mass drop, we can multiply to get the number of molecules in the molar mass. This is Avogadro's number.

 

[MedPR]

Did they make you calculate Oleic acid's molar mass based on its formula? That sucks.. doesn't oleic acid have 18 carbons a bunch of oxygens? Lame math.

 

[Dasypus]

No, they didn't make you, you just had to know doing so was possible.

 

[chiddler]

POE is really helpful for that particular question. Two non-polar and two-polar, but three non-h-bonders and one h-bonder. So the lonely guy (h-bonding one) is probably the answer.

i didn't think of Hbonding. I just thought of polarity, unfortunately.

i will remember to do this.

 

[chiddler]

For #1:

The volume of the drop times its density gives you the mass of the drop. If you know the volume of the drop and the volume of each molecule, you know how many molecules are in the drop.

Avogadro's number is the number of molecules in a sample that weighs in grams what the molecule weighs in AMUs. We know what oleic acid weighs in AMUs because we know its formula. So if we've got the number of molecules in any given mass drop, we can multiply to get the number of molecules in the molar mass. This is Avogadro's number.

oh i see. thanks this makes a lot more sense.

 

[MedPR]

i didn't think of Hbonding. I just thought of polarity, unfortunately.

i will remember to do this.

My experience with AAMC3-11 was a ton of H-bonding questions. It became the first thing I thought of on almost all of the bonding and solvent related questions. I don't know what I got on the real test, but on AAMC3-11 combined I got less than 10 gen chem questions wrong. The basic concepts really, really help.

 

[chiddler]

yes there were but most of them were so brain dead easy that it i didn't put much thought into most of them.

oh man. i think it was #8 or #9?

why does CH3-NH2 have a higher boiling point than CH3-CH3?

felt a little insulted that test makers thought so little of us to include this question lol

 

[MedPR]

yes there were but most of them were so brain dead easy that it i didn't put much thought into most of them.

oh man. i think it was #8 or #9?

why does CH3-NH2 have a higher boiling point than CH3-CH3?

felt a little insulted that test makers thought so little of us to include this question lol

Yea, some of them are pretty obvious. I think they give us easy h-bond questions so they can say that they gave us easy questions and not just hard ones. Too bad they pick the easiest topic to take the easy questions from.