Today, we dive into the second Chem/Phys passage from Blueprint MCAT’s Diagnostic. Listen in for in-depth explanations! We’re joined by Alex from Blueprint MCAT. If you would like to follow along on YouTube, go to premed.tv.
Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.
[02:24] Passage 2 (Questions 6 – 10)
Electrospinning is a technique used by industries in order to produce filters, membranes, and tissue scaffolds for implants. Electrospinning occurs when the electrical forces at the surface of a polymer solution overcome the surface tension and cause an electrically charged jet to be ejected. The solvent evaporates as the jet travels in the air, leaving behind charged polymer fibers that lay randomly on a metallic electrode (collector).
This is a classic science passage that throws loads of words at us. There are a few words here that we can point out that are tied into content areas that the MCAT expects us to have some knowledge on. These are words like electrical forces, surface tension, and charged polymer fibers.
[04:39] Paragraph 2
Fibers obtained may have diameters ranging from 0.05 to 5 µm. The small diameters provide high surface-area-to-volume and high length-to-diameter ratios. The process has applications in providing scaffolds for tissue engineering products.
Figure 1 Experimental setup for electrospinning
This paragraph means they have a lot of area on their surface relative to the amount of volume inside them. And they’re long relative to how wide they are. We can see here we’re creating a big, long, fibrous string. In figure 1, we get a graphical description of this process.
[06:43] Paragraph 3
The density, conductivity, surface tension, viscosity, and pH of the solution are controlled prior to electrospinning. Density is measured by determining the mass of a known volume of solution. These fibers can also be coated with proteins as a method of introducing biological molecules to the woven matrix. The encapsulation efficiency ε is determined by Equation 1:
Mm represents the measured mass of the proteins integrated with the fibers constructed, and Me is the theoretically expected mass of the proteins integrated with the fibers. Furthermore, the productivity, Ψ, of the process is calculated using Equation 2:
Here A is the total area of the fibers and tspin is the spinning time.
It’s an equation for encapsulation efficiency. And we get a mini-paragraph that describes it. Electrical efficiency is often work done by the electricity relative to the electricity used.
We get furthermore the productivity of the process is calculated using equation two, which looks like here this productivity term is calculated by the encapsulation efficiency divided by the area of the fibers multiplied by the spinning time.
Don’t over-interpret these terms because you can always just come back and figure it out more in-depth if it will be asked later in detail.'I would spend a lot of time puzzling out figures in equations in the passage only to realize the questions never mentioned them.'Click To Tweet
[10:12] Paragraph 4
An experiment was performed on 3 different fibers to determine their mechanical properties. A 10 N initial load and stress rate of +0.5 N/second were used. Results of the experiment are shown in Figure 2. The Young’s modulus was calculated by analyzing the peak stress that the sample was able to endure without deforming.
Figure 2 Results of a mechanical stress test performed on 3 fiber samples
We’re ultimately looking at their mechanical properties. Terms are highlighted in bold above. Again, if we get a question on it, we can spend the time and really puzzle out and see what it actually says.
[11:39] Question 6
What is the mechanical work done by the initial load placed on a fiber if the fiber is linearly deformed by 3 nm?
B.3 x 10-8 J
C.3 x 10-6 J
D.1.5 x 10-5 J
This is classic MCAT in the sense that it takes a straightforward physics concept, and applies it in such a strange way to this passage that it makes it incomprehensible. And so, this is asking what is the work done by a force through a distance?
In this case, the fiber is being linearly deformed, which means what’s the angle between the force that’s being applied, and the direction the fiber is stretching in. There is force being applied in the same direction that it’s stretching. So there’s the angle of separation between them as zero degrees and the cosine of zero degrees is one. So we can actually just completely ignore trigonometry here.
What this boils down to is just force multiplied by a distance which is to say 10 Newtons multiplied by three nanometers. Because this is asking for Joules (J), it does require that all of the units be in the form of SI base units (ex. kilograms, meters and seconds, Newtons and meters). So we can leave 10 Newtons as is. But we would have to convert three nanometers to meters, which the conversion factor for that is 10-9. This question reduces to 10 x 3 x 10-9, which is 30 x 10-9, that is to say, shifting over the decimal point three times 10-8. Therefore, B is the correct answer.
Correct Answer: B
[16:24] Question 7
Which of the following waves would have a wavelength larger than any standard electrospun fiber?
Alex says this is another classic example of how the MCAT phrases questions in a way that makes them sound much more difficult than they actually are. This question implies that you have to figure out the diameter of an electrospun fiber, and then have all of the wavelengths of all of these forms of light memorized.
But remember that the MCAT is a multiple choice quest check quite a multiple choice test. And so, only one of these answer choices can even be correct.
Alex rephrases this question as: which of the following waves has the largest wavelength? Because if there were two that had a wavelength larger than an electrospun fiber, both of them can’t be right. There will be three here that are shorter and one that’s longer. This question is just asking which which of the following forms of electromagnetic radiation has the longest wavelength.
Red and blue light are on the order of nanometers and X-rays are short. And so, microwaves are the larger ones.
Correct Answer: C
[20:18] Question 8
Assuming a mass of 0.04 g, what is the power exerted by the electrical forces on a fragment of fiber if the fragment has a velocity of 200 mm/s, 10 ms after ejection?
A.5 x 10-10 W
B.1 x 10-8 W
C.2 x 10-5 W
D.8 x 10-5 W
Alex says this requires a comfort with quick math that not very many people possess when they first start studying for this test.'Don't let a hard question stop you from answering an easier question at the end of the section.'Click To Tweet
This is a power question and the units of power is work per time, and the classic unit of power is the watt, which is a joule per second. Think of like a 60-watt light bulb, which consumes 60 joules of energy per second.
In this case, we have the quantities that we need from the passage to answer this. The fragment is moving at a certain velocity, a certain speed after rejection.
Remember that things have energy. If they’re moving at a velocity, they have kinetic energy. If it gained that energy over that 10 seconds, then the electrical forces are exerting an amount of energy in a window of time, which means we can calculate the power from that.
The formula for kinetic energy is:
K.E. = 1/2 m v2
where m is the mass of the object, and v is its velocity.
Based on the data above, its mass would be 0.04 grams. The velocity would be 200 mm/s, which once we convert that to meters, is 200 times 10-3 meters per second. And we’d have to square that.
Now, that we have its kinetic energy, we could divide it by the amount of time that it took to accelerate to that speed, which is to say 10 milliseconds, or 10 times 10-3 seconds. Once we do all of that math out, we’d end up with 8 x 10-5 Joules per second or 8 x 10-5 W.
Correct Answer: D'The diagnostic is not designed to be way harder than the real thing to scare people into studying more. But it's also not designed to gently ease you into the MCAT.'Click To Tweet
[26:52] Question 9
If all other factors are held constant, which of the following scaffolds would have the highest productivity?
A.Me = 1 mmol
B.Me = 2 mmol
C.Me = 3 mmol
D.Me = 4 mmol
If it’s the highest, it’s either going to be some sort of direct relationship or some sort of inverse relationship. And so the correct answer will lie on the extremes. Hence, Alex would eliminate B and C here immediately.
Efficiency and productivity are not quite the same thing. But they do relate to each other. Me is in the denominator. So of course, the lower that it is, the higher the deficiency would be. But productivity is the Equation two down below. But efficiency, that epsilon is in the numerator. So efficiency and productivity are directly related, which means that the relationship that Me has with efficiency is also true of its relationship with productivity.
Correct Answer: A
[29:39] Question 10
Which of the following best explains why, at a constant voltage, warmer solutions are ejected more quickly than cooler ones?
A.The droplets have smaller volume at higher temperatures.
B.The surface tension is lower for solutions at higher temperatures.
C.The electrical forces grow smaller as temperature increases.
D.The electrical forces grow larger as temperature increases.
Even if you don’t know anything about what’s going on in this passage, we know that when water temperature is higher surface tensions a little bit lower because water starts boiling, if you get high enough. And so, B is the only thing that makes sense here.
As the temperature increases, generally, surface tension tends to decline. Surface tension is a property of cohesion. Liquids tend to stick to themselves. And as you increase the temperature, as you increase the average kinetic energy of a liquid, they will have a propensity to stick to each other less, because of course, they become increasingly more likely to break away and become vapor. And of course, if you heat them up high enough, they’ll start boiling.
C and D are really tempting answers here. Because D, if it were true, would be correct. But electric forces on the MCAT are unaffected by temperature.
Correct Answer: B
[33:02] Final Thoughts
The importance of knowing everything on the MCAT is nil. You can’t know everything. The importance of reading comprehension, and using your little noggin up here and try to rationalize some things may make you wrong sometimes.'The MCAT is not a test of your content knowledge. It's a test of your ability to get MCAT questions correct.'Click To Tweet