# Quiz 5 - Totalscore 60/70=85.7143 .0 :70 1 StudentResponse...

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Total score: 60/70 = 85.7143% Total score adjusted by 0.0  Maximum possible score: 70  1. Which of the following has the lowest  vapor pressure at 25  ° C?  Student Response Value Correct Answer Feedback 1.  CH 3 CH 2 OCH 2 CH 3 2.  Ethyl-methyl-ketone 3.  Butanal  4.  CH 3 CH 2 CH 2 CH 2 OH 100%  General Feedback: The compound with the lowest vapor pressure is the one with the strongest intermolecular forces.  Since the hydrogen-bonding that dominates in 1-butanol is stronger than the dipole-dipole forces that  dominate in the other three cases, 1-butanol has the lowest vapor pressure. Score: 10/10  Comments: 2. The protonated form of derivatives of the guanidino group are weak acids.  If the p K a  for the methoxy derivative is 6.7,  what is the ratio of  protonated:unprotonated  form at physiological pH (7.4)?

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Student  Response Value Correct Answer Feedback 1.   5.0 : 1 2.   0.20 : 1 100%  3.   10 : 1 4.   0.10 : 1 General Feedback: At pH 7.4, a weak acid group with p K a  6.7  would be expected to be mostly in the deprotonated  (conjugate base) form.  We can calculate the exact ratio by rearranging the Henderson-Hasselbalch  equation: log([A - ]/[HA]) = pH - p K a , or log([HA]/[A - ]) = p K a  - pH, or [HA]/[A - ] = 10 (p K a - pH) Applying the values from the question: [HA]/[A - ] = 10 (6.7 - 7.4) [HA]/[A - ] = 0.20 Score: 10/10  Comments: 3. A pharmaceutical company is searching for a new compound to inhibit enzyme X by finding a molecule that binds in its  active site shown below. Based on what you have learned about intermolecular forces, which compound below would be  expected to bind enzyme X most tightly?
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