Couple of questions on 2009 test

[blankman321]

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so I bought the ADA 2009 exam and did pretty well overall but I wanted to know if anyone could explain the reasoning behind the following questions:

General chemistry:
#50
#54

QR:
#3
#4
#5

If anyone could give me any advice that would be greatly appreciated. I was not sure if i was allowed to post pics of the questions but if I am allowed to, let me know.

 

[paymaant]

so I bought the ADA 2009 exam and did pretty well overall but I wanted to know if anyone could explain the reasoning behind the following questions:

General chemistry:
#50
#54

Organic chemistry:
#72
#74
#76
#93
#99

QR:
#3
#4
#5

If anyone could give me any advice that would be greatly appreciated. I was not sure if i was allowed to post pics of the questions but if I am allowed to, let me know.

The title of this thread is misleading; you are asking more than just a couple of questions...

 

[blankman321]

The title of this thread is misleading; you are asking more than just a couple of questions...

sorry I guess I'll take out some. its just frustrating that there is no solution section they give our for the paper test.

 

[blankman321]

could anyone with the 2009 ADA test, at least show me to do question #3 and #4 on the math section? im really lost on those two

 

[Xo1991]

for #50, well you know that one of the colligative properties is freezing point depression, which follows the formula deltaT=ikm

i=1 since ethylene glycol is a nonelectrolyte
k=2, given (rounded from 1.86)
m= molality, in this case 2mols/1kg

so deltaT=(1)(2)(2)=4 degrees celsius, and you know that it is freezing point depression, so the freezing point goes from 0 to about -4, which gives answer D.

#54 Starts off by saying HCN is a weak acid, writing out the disassociation equation you get HCN <---> H+ + CN-. Ka expressions are products/reactants. Ka=([H][CN])/[HCN]

 

[Xo1991]

#3 is pretty simple, #5 just know the properties of an ellipse. Not sure how to do number 4

#3: convert those 1's into 7/7 to have a common denominator. so then the numerator becomes 10/7, then you are dividing that by the denominator, 5/7. Dividing a by a fraction is the same as multiplying by it's reciprocal, so it becomes (10/7) x (7/5) which equals 2.

#5...just look up the formulas for an ellipse and it's plug and chug from there.

 

[Xo1991]

Orgo:

#72 Not sure why they give that reaction first. Anyways oxymerc-demurc just adds an alcohol Markovnikov and doesn't go through a carbocation, so it doesn't have the potential to rearrange. The latter part doesnt really apply to this question, but when you add the alcohol to 'C', it adds to Carbon 2, creating a secondary alcohol

#74 D. Most thermodynamically stable is the conjugated diene.

#76 Recognize that the molecule is an amino acid. The carboxylic group has pKa~2, amine group has pKa~10. So at pH 7 the amine group will be protonated, while the carboxyl group is deprotonated

#93 well the grignard acts as the nucleophile, specifically the methyl group. Meanwhile the carbonyl carbon is the electrophile. So the methyl attacks, and the electrons of the double bond end up on the oxygen. Since it is asking for the intermediate, you know that the O must have a negative charge, while the grignard now only has MgBr+ since the Methyl took the electrons in order to act as the nucleophile

#99 I have no fricking clue.

 

[blankman321]

thanks so much! i wonder if some of these odd questions are trial questions.