What Makes Lab Technique Questions High-Yield?

Session 154

Today, we’re covering questions around a high-yield topic in O.Chem, particularly, lab techniques. Clara from Blueprint MCAT (formerly Next Step Test Prep) is helping us dissect each question!

Aside from this podcast, please do listen to The Premed Years Podcast to learn about what other ways to make yourself competitive. Learn about some information about shadowing, clinical experience, personal statement, letters of recommendation, and more!

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[03:18] General Tips and Tricks

We don’t usually anticipate a lot of questions about these techniques. In fact, O. Chem is the least tested subject on the MCAT. But with lab techniques, they often involve things like biochemical molecules or physiologically relevant molecules.

'You might end up with more organic chem questions than you expect because they might be asking them in a biochem sense.'Click To Tweet

Essentially, they’re really the same lab techniques you’ve learned in school like distillation, and the different separation techniques like thin layer chromatography. But be prepared for some physiological connection or biochemical connection.

[04:30] Question 21

Which solvent would be most suitable for the recrystallization of acetylsalicylic acid (aspirin) immediately after its synthesis? Note that the known melting point for acetylsalicylic acid is 136°C and the sample collected is displaying a melting point of 135.9°C.

Solvent 1, in which acetylsalicylic acid is insoluble at both 90°C and 15°C
Solvent 2, in which acetylsalicylic acid is highly (and equally) soluble at all temperatures
Solvent 3, in which both acetylsalicylic acid and its common impurities are fairly soluble at 15°C but insoluble at 90°C
None of the above; recrystallization ether would not work or is not necessary for the purification of this sample.

Clara’s insights:

The correct answer here is D.

Acetylsalicylic acid has to be recrystallized so B doesn’t make sense because it’s a solvent and it’s soluble at all temperatures.

The key to recrystallization is that we need to turn something into a solid material. To use it as a lab procedure, we need the best possible solvent to dissolve the solid in. Then it’s going to recrystallize and when it does, it typically has no impurities that it had before.

For this to happen, there are certain things to be true. For one, we need our solid to dissolve in it. If we put our aspirin in the solvent and it never dissolves in the first place then it can’t recrystallize either. Hence, A is wrong. Just think of it as generic high temperature and generic low temperature.

Answer choice C doesn’t have that problem because our aspirin is soluble at 15°C but then insoluble at 90°C.

That’s the opposite of the trend we typically see, wherein it’s relatively insoluble at cold temperatures. But then we can heat it up and it becomes soluble. Then we cool it back down and it drops out a solution as a solid.

Hence, C doesn’t make sense with regard to the temperatures. Additionally, it says both aspirin and its common impurities are fairly soluble. If its impurities dissolve in the same pattern that the aspirin itself dissolves, this is not absolutely perfect.

Recrystallization solvent is one where our desired solid dissolves but the impurities don’t. Or when we’re going to recrystallize it later, our desired solid turns back into solid but the impurities don’t. So the more our solvent treats our aspirin differently from its impurities, the better.

[12:27] Question 22

Vacuum distillation should be used to separate which of the following pairs of compounds?

(A) Lauric acid (BP=299°C) and propylene carbonate (BP=240°C)

(B) Acetone (BP=56°C) and ethyl acetate (BP=77°C)

(D) Formic acid (BP=101°C) and water

Clara’s insights:

The correct answer here is A.

Vacuum distillation is used to separate compounds that have very high boiling points.

For instance, if you have lauric acid with BP 299°C, we can never really get a distillation flask up to 299°C in the lab. It’s a little extreme.

Instead, we use a technique called the vacuum distillation where we typically lower the pressure of the apparatus. This lowers the boiling point.

So when asked about vacuum distillation, always pick the answer with the highest boiling points. There’s nothing about the difference between the two boiling points. But just look for the very high boiling points and so we use vacuum distillation to lower the boiling point.

On the contrary, fractional distillation is used to separate compounds with really similar boiling points. So when asked about fractional distillation, then D would be the answer as their boiling points are really close together.

[17:36] Question 24

A separatory flask contains two layers: water and diethyl ether. Aniline, benzoic acid, and phenol are dissolved in the ether layer. To isolate the benzoic acid, a student should add:

(A) a concentrated solution of KOH.

(B) a concentrated solution of HCl.

(D) additional water and diethyl ether.

Clara’s insights:

The correct answer here is C. This is about extraction. And extractions are really confusing to a lot of students.

For the MCAT, we always either want to separate out some sort of acid or base. There are other types of extractions but the MCAT doesn’t ask about them.

Therefore, you just need to figure out whether you’re dealing with an acid or a base.

For this question, you’re dealing with an acid.S o the next thing you need to know is that if you want to separate out an acid, the easiest way to do it is to be protonated.

This is true across the board. Things will tend to be really soluble in water if you make them charged. And here, our aniline, benzoic acid, and phenol are all in the other layer. They’re not in the water layer.

Now, if you want to move one into the water layer, you have to get all separated by itself by making it charged.

So whenever you’re dealing with any sort of extraction, you want to deprotonate an acid to make it charged and it will make it go into the water layer.

In this case, you need to add a base to separate the acid. And only answer choices A and C are talking about bases. To decide between these two, you need to look at what else is in the ether layer – aniline and phenol.

Interestingly, phenol is an acid. Just like benzoic acid, it’s weaker but it’s still an acid. You don’t want to add a super-concentrated base otherwise it will separate out both of the acids. And you won’t have benzoic acid by itself, but you will have benzoic acid and phenol.

Instead, you want to add a really weak acid like a dilute solution of ammonia. Then you can just separate out the first acid that will go into that water layer, which is benzoic acid.

[22:05] Dealing with Multiple Pieces of Information

There are plenty of MCAT questions where you need multiple pieces of information and you need to integrate them.

Most often, there will be some way you can reason around it. For instance, if the question requires three pieces of information, you can typically get it right if you know two of those pieces. Then you also have to use some sort of scientific common sense.

In this case, say you don’t really know what phenol is, then common sense would tell you that A seems pretty extreme. It’s a concentrated solution of KOH which is a strong base. Whereas C is the opposite which is a dilute solution dropped in slowly.

In the MCAT, it’s best to just go with less extreme answers. So this thought process can help you get through this question.

[23:45] Blueprint MCAT (formerly Next Step Test Prep)

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