This week we’re staying away from one specific section and are doing a grab bag of discrete MCAT questions. Take a listen and follow along with us.
[01:22] Mental Flexibility
Bryan mentions that the real MCAT really requires you to be flexible for your brain to change gears quickly on the fly under timed conditions. So for this week and the next couple of weeks, we intend to jump from topic to topic in order to flex that mental agility.
A real life-example of how this works is golf. When you go to the driving range, you usually use one club over and over again and then switch to another club and go over and over again. But that’s not how you play golf. It’s one club, one time, another club, another time. So you need to practice how you’re going to perform.
[02:17] Question #32:
A researcher carries out a column chromatography at physiological pH, using a stationary medium with a net positive charge. If a solution containing the following oligopeptides is poured into the column, which oligopeptide will most likely be found in the first fraction collected?
- (A) DDGE
- (B) EILD
- (C) KRVV
- (D) VEGP
The trick here is recognizing the charges amino acids have at physiological pH. This is something the MCAT is definitely going to expect us to know at the top of our head. Recognize that the medium has a positive charge so anything that’s negative is going to stick to the stationary medium and just stay there.
The question wants the first fraction that falls through this column. You should remember that the amino acids D (aspartate) and E (glutamate) are the two that have a negative charge at physiological pH and D and E, aspartate and glutamate go the same order alphabetically.
Answer choice (A) DDGE is a whole bunch of negative and (B) EILD where E is a negative and (D) VEGP where E is a negative. So three of the four answer choices have a negative amino acid and that negative amino acid would stick to the positive charge and stay in the column. Hence, the right answer for this one is answer choice (C) KRVV because K and R are positive and V is neutral so it would just fall right along the positive so it would repel each other.
[05:00] Understanding Amino Acids
For each amino acid, you would need to understand specific things such as name, the three-letter and one-letter abbreviations, whether it’s charged, and broadly what its chemical behavior is like (polar versus nonpolar). Finally, you should be able to draw all 20 of the standard canonical amino acids from memory. You don’t have to remember the weird ones like the selenocysteines as they’re much less likely to come up.
[06:16] Question #48:
Which of the following solvents would lead to the fastest SN1 reaction:
- (A) N-hexane
- (B) Benzene
- (C) Tetracholoromethane
- (D) Propenol
You can always play the game of which one of these doesn’t belong when you get one of these orgo questions with four different molecules for choices. Hexane and benzene are fairly generic nonpolar solvents which are hydrocarbons. Benzene is a ring and N-hexane is a chain. There is no reason to pick one over the other so you can cross both of them out.
For choices (C) and (D), chlorine is electronegative so that would make a polar bond except when you have tetrachloromethane. All four chlorines pulling on the same carbon. Technically, the bond is polar but the molecule is not because it’s kind of a four-way tug-of-war whether they will all cancel each other out. Hexane, benzene, and tetrachloromethane are all nonpolar solvents. You can cross them all off together.
Propanol alcohol is not just polar but protic. It’s got that OH group and the alcohol that makes it very water-like in its chemical behavior. If nothing else, that’s how you would get to the right answer there.
[08:07] Understanding SN1 and SN2 Reactions
You should walk into the test being very comfortable with SN1 and SN2 reactions and knowing that SN2 prefers an aprotic so no water and no alcohol whereas SN1 prefers a protic solvent.
When it comes to named organic chemistry reactions, when they switched over to this current format of the MCAT in 2015, they switched almost all of them. But SN1 and SN2 are the old die-hard’s and Bryan says when humanity has gone the way of the dinosaurs, there’s still going to be a standardized test asking about SN1 and SN2 somewhere.
[09:03] Question #46:
A single sports fan is capable of yelling at an intensity level of 80 decibels from a given distance. If 10,000 similar fans were all yelling from the same distance, which of the following will be the closest to the observed intensity level?
- (A) 84 decibels
- (B) 120 decibels
- (C) 160 decibels
- (D) 320 decibels
Decibel is a measure of intensity which is a measure of loudness. A crowd of 10,000 screaming at you is going to be louder than one person screaming at you. So it’s going to be a number bigger than 80 and all of the answer choices are bigger than 80.
In this current version of the MCAT, even if it’s a physics question, you can reason from biology. You want to walk into the test knowing that 80 decibels could be a human screaming very loudly and the threshold of pain to the point where the noise itself starts to actually cause pain in your ear is somewhere in the 130-140 decibel range. Imagine a stadium full of people all shouting, that’s going to be pretty loud.
Answer choice (A) is only a little more than 80 so that doesn’t sound right for an entire stadium full of people shouting at you. Then you have to decide whether it’s loud enough that it’s going to start physically hurting just hearing a stadium full of people cheer, maybe yes, maybe not. Answer choice (B) is below the threshold of pain while (C) is above the threshold of pain. Choice (D) would be like shoving your head in the jet engine. It’s massive and that could be causing earthquakes.
So you could edge your way close to the right answer here even if you couldn’t remember the equation. Although you should know the decibel equation which is a logarithmic scale. So going from one person up to 10,000 people, you recognize 10,000 as 104 times bigger than the original intensity level.
But the thing about the decibel scale that makes it a pain in the butt to remember, unlike other logarithmic scales like pH or pK or whatever, the decibel scale is ten times the log of the intensity or the intensity difference. So the 4 becomes a 40 and our 80 decibel goes up to a 120 decibels. It could be a pain in the butt but you have to remember that decibel equation is ten times the log of the intensity.
[13:30] Final Thoughts
Our episode today is basically all about things you need to memorize for the MCAT. Know your amino acids. Know your SN1 and SN2 stuff. Know your decibel equation. Don’t forget that even if you can’t remember the exact content, you can edge your way closer to the right answer by thinking in terms of the biology or playing which one doesn’t belong.
[14:15] Next Step Test Prep
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