MCAT Question of the day... wrong AGAIN?

JLeBling

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I've been doing http://mcatquestionoftheday.com/ every morning as a warm up. And I am fed up with seeing all these mistakes! Am I crazy, or is something not right here?

The isoelectric point of glycine is 6.0. When glycine is in a buffer with a pH of 6.0, which form predominates?


A. H3N+-CH2-COO- (49%) [Isn't it supposed to exist as a zwitterion in an aqueous solution?
B. H2N-CH2-COOH (21%)
C. H3N+-CH2-COOH (21%)
D. H2N-CH2-COO- (9%)

Correct Answer: B. H2N-CH2-COOH

Recall that you can use isoelectric point (PI) to predict an amino acid’s charge at any given pH. Specifically, if PI < pH, then this means that pH is high -- which means that [H+] is low, and charge is negative. Likewise, if PI > pH, then this means that pH is lower — which means [H+] is high and charge is positive. In this case, PI = pH, which means that charge is zero (the amino acid is neutral). Thus, the answer is B since this is the only form without charge.
 

Leonardo Noto

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I've been doing http://mcatquestionoftheday.com/ every morning as a warm up. And I am fed up with seeing all these mistakes! Am I crazy, or is something not right here?

The isoelectric point of glycine is 6.0. When glycine is in a buffer with a pH of 6.0, which form predominates?


A. H3N+-CH2-COO- (49%) [Isn't it supposed to exist as a zwitterion in an aqueous solution?
B. H2N-CH2-COOH (21%)
C. H3N+-CH2-COOH (21%)
D. H2N-CH2-COO- (9%)

Correct Answer: B. H2N-CH2-COOH

Recall that you can use isoelectric point (PI) to predict an amino acid’s charge at any given pH. Specifically, if PI < pH, then this means that pH is high -- which means that [H+] is low, and charge is negative. Likewise, if PI > pH, then this means that pH is lower — which means [H+] is high and charge is positive. In this case, PI = pH, which means that charge is zero (the amino acid is neutral). Thus, the answer is B since this is the only form without charge.

Wow, this is making me dig way back in the old dusty, spider-infested info closet. But since no one else has responded, here's my two cents. I think that the problem you're having boils down to this--yes, the zwitteron form predominates at physiologic pH--but 6.0 is not physiologic pH (~7.4 is)!!!

Dr. Leonardo Noto
www.leonardonoto.com
 

Jepstein30

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Jun 29, 2011
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I've been doing http://mcatquestionoftheday.com/ every morning as a warm up. And I am fed up with seeing all these mistakes! Am I crazy, or is something not right here?

The isoelectric point of glycine is 6.0. When glycine is in a buffer with a pH of 6.0, which form predominates?


A. H3N+-CH2-COO- (49%) [Isn't it supposed to exist as a zwitterion in an aqueous solution?
B. H2N-CH2-COOH (21%)
C. H3N+-CH2-COOH (21%)
D. H2N-CH2-COO- (9%)

Correct Answer: B. H2N-CH2-COOH

Recall that you can use isoelectric point (PI) to predict an amino acid's charge at any given pH. Specifically, if PI < pH, then this means that pH is high -- which means that [H+] is low, and charge is negative. Likewise, if PI > pH, then this means that pH is lower &#8212; which means [H+] is high and charge is positive. In this case, PI = pH, which means that charge is zero (the amino acid is neutral). Thus, the answer is B since this is the only form without charge.

I think they ARE wrong. Their reasoning is correct though. If pH = pI, you'd expect no charge on the molecule.

However, COOH has a pKa of around 2.. so we know that it will be deprotonated because pH > pKa (i.e. its in a basic environment relatively). NH3 has a pKa of 9.. so we know that it will be protonated since pH < pKa (i.e. its in an acidic environment relatively). The zwitterion fits that critera and also has no overall charge, just like we'd expect at pI. Even the answer explanation skips over the fact that the zwitterion is neutral because it says B is the ONLY form without charge.. obviously incorrect.

Yea, i'm pretty sure they are incorrect..
 

Leonardo Noto

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rjosh33

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I've been doing http://mcatquestionoftheday.com/ every morning as a warm up. And I am fed up with seeing all these mistakes! Am I crazy, or is something not right here?

The isoelectric point of glycine is 6.0. When glycine is in a buffer with a pH of 6.0, which form predominates?


A. H3N+-CH2-COO- (49%) [Isn't it supposed to exist as a zwitterion in an aqueous solution?
B. H2N-CH2-COOH (21%)
C. H3N+-CH2-COOH (21%)
D. H2N-CH2-COO- (9%)

Correct Answer: B. H2N-CH2-COOH

Recall that you can use isoelectric point (PI) to predict an amino acid’s charge at any given pH. Specifically, if PI < pH, then this means that pH is high -- which means that [H+] is low, and charge is negative. Likewise, if PI > pH, then this means that pH is lower — which means [H+] is high and charge is positive. In this case, PI = pH, which means that charge is zero (the amino acid is neutral). Thus, the answer is B since this is the only form without charge.

So my girlfriend is taking the MCAT in May and is doing these Question of the Day things as well. After reading your post we went back and looked at her version of the question and they say the correct answer is A. The explanation is similar to yours but a little different, with the big difference, of course, being the change in the listed answer. So I guess at some point they corrected their error, but still send the incorrect version to some students. Awesome.

From what I can tell, these questions are alright for the most part, but at times their answers are debatable, or just flat wrong (e.g., the other day they claimed a reaction at 500 deg. Celsius would favor SN1 over E1). I couldn't tell which company produces these questions, but I'd be kinda pissed if I had paid for their product. As I understand it though, these questions are free, so tough to get too upset about it.
 

JLeBling

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Here is another one...

Boyles law is a special case of the ideal gas law in which P1V1 = P2V2, stating that the volume of a gas is inversely proportional to pressure at a constant temperature. Which of these gases correctly demonstrates Boyles Law?

Screen-Shot.png


A. A (52%)
B. B (19%)
C. C (26%)
D. D (3%)
Correct Answer: A. A.

The correct answer is A. Looking at the graph for A, we see the pressure at point 1 is 10 kPa, and at point 2 is 25 kPa. The volume at point 1 is 50L. 50 x 10 = 500, and following Boyles Law, the volume at point 2 must be 500/25, or 20l. The other graphs do not show this correctly.

Aren't inverse relationships supposed to be curved? Like so:
AAAUAYB0.JPG


The exception being if it is plotted on a log scale? Or is this just an aesthetic thing?
 

NuttyEngDude

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haha that is what i was thinking after doing this problem but you only have two data points.... so they are correct to draw a single line between two points. in doing this problem i approached it qualitatively by just choosing the one with highest variance (A) that would reflect the curve, but the best solution is the one they provide which plugs in values.
 

Temperature101

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Here is another one...

Boyles law is a special case of the ideal gas law in which P1V1 = P2V2, stating that the volume of a gas is inversely proportional to pressure at a constant temperature. Which of these gases correctly demonstrates Boyles Law?

Screen-Shot.png


A. A (52%)
B. B (19%)
C. C (26%)
D. D (3%)
Correct Answer: A. A.

The correct answer is A. Looking at the graph for A, we see the pressure at point 1 is 10 kPa, and at point 2 is 25 kPa. The volume at point 1 is 50L. 50 x 10 = 500, and following Boyles Law, the volume at point 2 must be 500/25, or 20l. The other graphs do not show this correctly.

Aren't inverse relationships supposed to be curved? Like so:
AAAUAYB0.JPG


The exception being if it is plotted on a log scale? Or is this just an aesthetic thing?

You are correct. These two websites have been wrong lately.
 

jbrharris84

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Mar 1, 2012
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Ya they've been missing the boat for the past few weeks...

Does anybody remember the one with the momentum of the sneeze droplet? LOL I wish I could find it. Great comedy.
Agreed. I unsubscribed a long time ago and visit every once in a while for some entertainment. Other questions of the day have treated me much better like M Prep and Kaplan.
 
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