Session 70

Chemistry on the MCAT is hard for a lot of premed students. Today, I and Bryan of Blueprint MCAT (formerly Next Step Test Prep) will go over some chemistry questions and break down each of them to give you a head start.

[01:20] Amino Acids

Question 10: At pH 7.3, what is the bond order of the shortest bond to Oxygen in Glycine?

Bryan’s Insights:

You should recognize that pH where 7.3 to 7.5 is the physiological pH. Glycine is an amino acid. A physiological pH acid exists as zwitterions. The amino part is positively charged, NH3+. And the acid part is negatively charged, COO-. This is the memorization side of it.

Then you have to apply what you know about resident structures. The acid part of an amino acid is usually drawn as carbon double bonded to oxygen with another single bond to an O-.

The potential trap here is just immediately picking two, that the strongest and shortest bond is a double bond (C=O). But you want to remember that COO- exists as a resident structure.

So the single and the double bonds are actually residents with each other so the bond order is 1.5. In fact, that’s the only bond between carbon and oxygen in Glycine. So 1.5 would have to be the right answer whether they ask for the shortest or the longest.

Bryan recommends that what you do with flashcards and study sheets for amino acids is draw the physiological pH. But in the end all that really matters is consistency. As long as you’re studying the same way every time and know the underlying principles, you’ll be okay.

[04:05] Lab Techniques

Question 30: Commercial preparations of the compound Captopril requires that it be separated from its enantiomer. Which of the following techniques would best accomplish this?

• (A) Fractional distillation
• (B) Thin layer chromatography
• (C) Chiral resolution
• (D) Recrystallization

Bryan’s Insights:

This is a question around lab techniques which are very important for the MCAT. So you have to know how all of these separate various molecules.

Fractional distillation is based on boiling point like all distillation. Thin layer chromatography is based on polarity whether something likes to stick to the stationary phase or move along with the mobile phase.

Chiral resolution has something to do with isomers pulling them apart. And recrystallization has to do with solubility. This could be crystallization at the bottom of the beaker or it stays dissolved.

In this case, you’re separating enantiomers, a particular kind of isomer. Chiral resolution, therefore, is how you would separate them.

[05:54] Ideal Gas Assumptions

Question 57: Which of the following is not a characteristic of an ideal gas?

• (A) The average kinetic energy of the gas sample depends on the mass of the molecules.
• (B) Collisions between gas molecules are elastic.
• (C) There are no attractive and repulsive forces between gas molecules.
• (D) Gas particles have a volume of zero.

Bryan’s Insights:

My guess here is that gas in a volume of zero just sounds weird to me so I would pick that one.

Bryan explains that (D) is an assumption of ideal gas. You’d assume that the molecules themselves have no size. So when the gas is filling up the balloon or the beaker, all of that space is just filled with the space between the molecules.

In this case, the ideal gas law just have some basic assumptions. So the answer choices B, C, and D are three of the big classic assumptions that the gas molecules have no forces on each other. Either attractive or repulsive, they take up no size. And that any collisions are perfectly elastic.

The ideal gas law assumes that the gas particles are these perfectly spherical little ping pong balls bouncing off each other perfectly elastically. The ping-pong balls themselves take up no space. So it seems a little silly but useful assumptions about the gas molecules themselves.

So if you’re not sure about kinetic energy and you couldn’t remember the equation for that, just set aside (A) for a moment. And see if all the others are ideal gas assumptions. Then come back to A, which is the right answer.

[08:30] Blueprint MCAT (formerly Next Step Test Prep)

Next week, we talk about biology questions. Meanwhile, share this podcast to your friends, classmates, and everyone. Also, check out Blueprint MCAT (formerly Next Step Test Prep)‘s MCAT course. They’re known for their one-on-one tutoring.

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Links:

Blueprint MCAT (formerly Next Step Test Prep) online course