MP 60: Breaking Down a Grab Bag of Science Questions for the MCAT

Session 60

Today, we're diving into discrete MCAT questions from various subjects. Follow along and have a listen. Don't forget to subscribe and tell your friends. Last week, I and Bryan of Next Step Test Prep talked about this last week. Even something like Physics can touch all these other disciplines. So it's great to also do a grab bag and do a whole bunch of different disciplines in the sciences to practice that quick shifting of mental gears that the MCAT demands. Please listen to all our other podcasts on MedEd Media.

[01:46] Physics Question

Question #30: The transition from a vapor phase directly to a solid form is known as:

  • (A) Condensation
  • (B) Freezing
  • (C) Sublimation
  • (D) Deposition

Insights:

Sublimation is from solid to gas. A classic example of this is dry ice. This one going from a vapor to solid is deposition. You deposit down onto a solid. Condensation is vapor into a liquid like drinking ice into a hot day or the morning due. And obviously, freezing is from liquid to solid.

[02:49] Biology Question

A researcher analyzing a genome measures the cytosine composition of 19%. What is the expected adenine composition of this genome?

Insights:

The C and A have to add up to 50% because the G and T will add up to the other 50%. So you get to an answer of 31%. Many months ago, we handled a tricky question because it was only a single-stranded DNA segment. So you always have to be careful. This question said the whole genome so it means it's double stranded. This said, you can do the math we just did above.

[04:00] pH Question

Question #47: The gastric juices in the stomach have a pH of approximately 2. What is the hydroxide ion concentration in this solution?

  • (A) 0 molarity
  • (B) 10 to the minus 2nd molarity
  • (C) 10 to the minus 7th molarity
  • (D) 10 to the minus 12th molarity

Insights:

The MCAT is going to expect you to remember the pH equation. pH is the negative log of the H concentration. You want to remember that the shortcut when you're doing this pH or negative log problems. You take the exponent on the 10 and bring it down and get rid of the negative sign. So for 10 to the minus 7, the negative log of that is just 7. Take the minus 7 and bring it down and just get rid of the minus sign. So if your H concentration was 10 to the minus 7, that's a pH 7 solution. In this case, the gastric juice have a pH of 2. This means that the H concentration is 10 to the minus 2. But remember to answer the exact question they ask. They didn't ask for the proton or the H concentration. They specifically asked for the hydroxide ion. Remember that the hydroxide ion is OH minus. So it's kind of the converse of the acid. If you have a 10 to the minus 2 of your acid concentration, they have to multiply out to ten to the minus 14. Your acid times your base has to get you to 10 to the minus 14. Your pH plus your pOH has to add up to 14. In this case, if you're going really quick and rushing, you would be led to the wrong answer choice (B). Because it's not the answer to this question. The questions asks about the hydroxide ion, not hydronium ion. Hydroxide ion is answer choice (D).

[06:28] Just Answer the Question Asked

This is a very common extra little trick that when you're doing acid-base chemistry, make sure you solve for the acid or the base. Answer what they actually ask you for rather than just the first thing you calculated. Bryan who has been doing this for sixteen years, he would still sometimes be working with a student and make a mistake. Then they both get all twisted around. So even while working together, they get confused on a problem. They stop, take a breath. And just realize, that you just need to answer the question they asked.

[08:25] Physics Question

An unknown fluid has a specific gravity of 0.75. What is the volume of 22.5 kg of this fluid?

  • (A) 10 liters
  • (B) 20 liters
  • (C) 30 liters
  • (D) 40 liters

Insights:

The right answer here is 30 liters. But just be careful and don't fall for the trap of just going right to this or that number. Always be attentive to every MCAT question. Be careful but don't assume they're out to trick you. Assume that they're just asking you exactly what they asked you. So specific gravity 0.75. If you know what specific gravity is, that is essential. They give you a mass and they want you to convert to the volume. You have to walk into the test knowing the specific gravity of water (this is by definition) which is 1. And water has density of 1 gram per ml or gram per cubic centimeter. Or it has a density of 1 kg per liter. These are all facts you have to walk into the test knowing. In this question, you see 22.5 kg and you would go 22.5 liters of water. That is your baseline answer. But this fluid only has a specific gravity of 0.75. It is less dense than water. And if it's less dense, it takes up more space. That said, your answer has to be bigger than 22.5. So it has to be 30 or 40. But 0.75 is still pretty close to 1. It's not like you have to double it up to 40. Without even doing any math, just using your gut instinct to it, 30 is the right answer. And it turns out the math is actually really simple since you just divide the numbers. 22.5 divided by 0.75 gives you 30 and this is the right answer.

[11:20] Use Your Imagination

You can do the whole unit conversion. But Bryan is much more a fan of physics in a way that you just imagine or think through whether it's a big or small number or does it get bigger or smaller. Is it something heavy or light? Throw the object straight up in the air and have a fall right down. It's a matter of using your imagination a little bit on the MCAT.

[12:18] Next Step Test Prep

Check out Next Step Test Prep on their products and services to help you on your MCAT journey. There are lots of ways to prep for the MCAT like a do-it-yourself course or a live online course. Get a tutor or go through a summer intensive. There are books and full-length practice exams that you can look at too to help you on your journey. Next Step Test Prep offers all of these. Use the promo code MCATPOD to save some money off their offerings.

Links:

Next Step Test Prep MedEd Media

 

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