2009 DAT #76, #80

[emitpeels]

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Can anyone help answer these problems?

Number 76 asks for the structure of glycine at ph 7. I thought that since this is neutral PH, both groups would be non charged. However, the answer has the amino group charged as +1 and the carboylic end as -1. Can someone explain why?

Number 80 asks to describe a bond angle on a carbon that is clearly sp3. I answered 109.5, however, the given answer is said to be 120 degrees. Anyone have any idea why?

I would normally post a picture, but I'm afraid to get sued by posting a pic xD

EDIT: I see my error in number 76. I am still unsure about number 80 though....

 

[FeralisExtremum]

I can't see number 80 and I don't have the test you are referring to, but is there by any chance resonance that could be happening on the carbon in question? If so, the hybridization would be considered sp2 rather than sp3.

 

[DogeDDS]

Charge on the different groups depend on the pKa. Amino has a pKa ~9.5 and Carbox. has one ~2. Since the pH is greather than (more basic) than COOH's pKa, it'll be deprotonated, however, the pH isn't basic enough for the amino group to get deprotonated as well.

For # 80, I definitely see an sp2 carbon. It's double bonded to 1 carbon, 1 hydrogen, and another carbon. I think you're looking at the first carbon and not the one labeled with the arc labeled as angle 1

 

[emitpeels]

I'm looking at it right now, there is no double bond. The carbon is connected to to three carbons and one H.

 

[emitpeels]

Here is the picture.

 

[sjv]

I'm looking at it right now, there is no double bond. The carbon is connected to to three carbons and one H.

Haha, I have this one, there seems to be 2 versions.

 

[emitpeels]

Man that pisses me off. I started at this problem for so long trying to figure out what I was doing wrong. That scares me, I wonder how many other errors there are..... Thanks for clearing this up!

 

[sjv]

Man that pisses me off. I started at this problem for so long trying to figure out what I was doing wrong. That scares me, I wonder how many other errors there are..... Thanks for clearing this up!

There are a few: http://forums.studentdoctor.net/threads/2009-dat-sample-test-errors-confirmed-by-ada.828825/

 

[emitpeels]

I've seen that list. The fact that this problem is not included makes me uneasy.

 

[DogeDDS]

No, there are a good number of errors unfortunately. That thread doesn't catch all of them. Some answers are debatable too. I remember a question in particular:

Developing organisms have negative entropy, and what do they use to aid this process?

A. Enzymes
B. Violating Second Law of thermodynamics
C. Using external source of energy
D. Concentrating their molecules and reverse the flow of energy
E. Creating Energy

The answer key said E but how can you create energy when the Law of Thermodynamics SPECIFICALLY states energy cannot be created or destroyed. Some people argued that you can create energy through making ATP but to get ATP, you had to couple it's formation with another energy converting process like photosynthesis (external light energy) or respiration (external food energy), essentially stating C. Luckily, there aren't AS many errors in the OC section.

 

[emitpeels]

For my answer sheet, the answer is listed as C for number 19.

 

[DogeDDS]

Guess they fixed it, but made sure to include other errors. I love it

 

[SLDENTAL]

Can you post a pic of number 76, I curious to see format of that problem!

 

[SLDENTAL]

So if the pH is greater than pka it will be deprotonated, why is that?

Charge on the different groups depend on the pKa. Amino has a pKa ~9.5 and Carbox. has one ~2. Since the pH is greather than (more basic) than COOH's pKa, it'll be deprotonated, however, the pH isn't basic enough for the amino group to get deprotonated as well.

For # 80, I definitely see an sp2 carbon. It's double bonded to 1 carbon, 1 hydrogen, and another carbon. I think you're looking at the first carbon and not the one labeled with the arc labeled as angle 1

 

[emitpeels]

This is the easy way I think of it. If the ph is lower than the functional group's pka, there will be a higher concentration of H+, thus, the group would be protonated (NH3+,COOH). If the ph is greater than the pka, there will be less H+, thus, depronated (NH2, COO-). Hope that helped.


Can someone answer this gen chem question for me? I'm stuck >_<.

 

[DogeDDS]

The answer is D. I put A initially but I realized that's only true IF the conditions are at STP.

You can look at the ideal gas law PV=nRT and derive any relation from anything else. If the two things you are comparing are on the same side of the equal sign, for example P and V, then they are inversely related. ; if they are on opposite sides, like V and T, then they are proportional.

 

[emitpeels]

Thats what I thought. I was thinking of "n" in PV = nRT as opposed to V.