Blueprint MCAT Full-Length 1: Passage 1 — Saltwater Chemistry

Session 182

In today’s episode, Phil and I embark on our journey through Blueprint MCAT full-length one. We’re “diving” into passage one and talking saltwater chemistry.

We’re joined by Phil from Blueprint MCAT, formerly Next Step Test Prep. You can either listen here on this podcast or on my youtube channel premed.tv, as we go through the full length one question by question, passage to passage.

Go sign up for Blueprint MCAT’s diagnostic to get a copy of their full-length one for free. And so after you take your full length, you can go to the YouTube channel, you can listen to this podcast, you can really go through the questions with us and find out where you went right, where you went wrong, and much more.

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

[04:27] Passage 1, 5 Questions

A hyper-sailing body of water contains high concentrations of sodium chloride (salt) and other water-soluble ionic compounds such as calcium sulfate (gypsum). The salt levels exceed those found in ocean water which contains 3.5% sodium chloride by mass and are often associated with flora and fauna that are specifically adapted to these extreme conditions. There’s considerable interest in species that can survive under such conditions because they may represent conditions for life on other worlds.

The best known hyper-sailing lakes are located in the hydrologically isolated environments, in which water inflow is in equilibrium with evaporation. Examples include the Dead Sea (DS) in the Middle East and the Great Salt Lake (GSL) in Utah. Salt concentrations in the Dead Sea are greatest in the southern section and approach saturation. There’s like talking about like Saturation levels. So I’m thinking like Aqs KSP is expecting maybe to see a question about this.

Salt concentrations in the GSL range from levels similar to ocean water up to saturation. Swimming in salty water is a significantly different experience than swimming in freshwater.

Phil’s thoughts: As you’re going through this everything, so far it has screamed chemistry, solubility saturation stuff. It mentions swimming so like buoyancy. There might be some physics questions coming down the line.

The average human body has a slightly greater density of 1.01 kilograms per liter than freshwater and people must usually swim to stay afloat in a body of freshwater like Lake Michigan. However, adding salt water increases the density of the liquid resulting in an increase in the buoyant force. The salt also affects other properties, including colligative properties such as reductions of vapor pressure, melting point, and solubility of gases.

Phil’s thoughts: Colligative properties are also coming down the line here. It’s really important as you’re reading a passage to realize what stuff you need to pull from it and what kind of questions you would expect to see. Because that makes it easier for you to anticipate what the exam writers are trying to do.

[07:34] Figure Description

A figure shows the solubility row in relation to temperature for a bunch of different salts. We have sodium nitrate, calcium chloride, lead nitrate, potassium nitrate bottom nitrates going on here, calcium dichromate. There’s kind of like a big mix of them. It seems like a lot of lines just crossing over. For the most part, all of these tend to be really greatly increasing.

As the temperature goes up, the water becomes more soluble for these. One thing that jumps out at you here is that calcium chloride shoots off the top of the chart. Then cesium sulfate seems to be the only one that goes down in solubility as temperature goes up..

Phil’s thoughts: Sodium chloride, which is probably the most well-known salt, is increasing but it is kind of flat. So that means that water at 80 degrees doesn’t have that much higher of a saturation point than water at 20 degrees. Versus for these other things, you can triple or quadruple the amount of salt in the water and in those temperature ranges.

Another figure shows a graph of pressure and temperature comparing solid, liquid, and gas. The solid to liquid graph keeps going like a straight line forever. But the liquid to vapor, or the liquid to gas usually ends in a point, called the critical point. We call it fluid at that point because the liquid and gas are both fluids.

[11:03] Continuation of Passage

Hyper-sailing bodies of water tend to be fairly sterile, containing only highly adapted forms of life. Brine shrimps are the most notable aquatic life form in the GSL (Great Salt Lake). The shrimp feed on algae by mastication using mandibles to create a bolus, which after injection into the mouth travels down the equivalent of the esophagus to the stomach.

Phil’s thoughts: There’s a little bit of crossover into physiology here.

In addition, anaerobic halophiles can be found, namely, formative sulfate-reducing homo acidogenic, phototrophic, and meth-antigenic bacteria.

Phil’s thoughts: This is just one of the ways for the MCAT to ratchet up the difficulty because MCAT students are smart. And it is very hard to separate them sometimes. So they’ll ratchet up the difficulty by using language, that’s a little bit more complex or a little bit less comfortable for students. So instead of light, they’ll say radiation, which is just light.

[12:52] Question #1

What’s the approximate solubility of sodium chloride in ocean water if the density of ocean water is 1.028 kilogram per liter.

Answer choices:

(A) 0.026 M

(B) 0.62 M

(D) 9.6 M

Breakdown of the Question:

They’re asking specifically about ocean water. There was something mentioned in the second sentence of the first paragraph. It says it contains 3.5% sodium chloride by mass.

This is where we start to utilize like we’re on question one. We’re already utilizing these little extra things that the MCAT gives us access to the periodic table. We can figure out the money. We can pull up the periodic table and look.

Convert to grams per mole

Looking at the periodic table, chloride is 35 grams, sodium is 23 grams. You can do some rounding because this is the MCAT. If the answer is 27, and you got less than 30, that’s good enough for the MCAT. 23 (Sodium) + 35 (Chloride) is around 60. Somewhere around there, about 60 grams per mole.

Find the actual grams of sodium chloride

Then take that the density of water, which is 1.28 kilograms per liter, multiply that by 3.5%.

Then convert to moles per liter

The Easy Way: Rounding Off

Most students are pretty comfortable with the idea that water is one kilogram per liter, just like freshwater, nothing else in it.

So you can say there are about 28 grams in this because the water is 1.028 kilograms per liter. So water counts for one of that. And the 0.028 is the sodium chloride. Now that’s actually like a pure number. But once again, we’re on the MCAT, we can do some ballpark math.

And so if sodium chloride is about 60 grams, and I have like 28 grams of it, it should be like about half a mole. Looking at the answer choices, B is close to half at 0.62M. So the correct answer is B.

[17:13] Round Things Off to Make It Easier

In undergrad, you’re punished for rounding. But on the MCAT, all of a sudden, that becomes useful and perhaps crucial on some questions to like learn because it saves time. You don’t have a calculator so you want to avoid doing calculations on a scratch paper.

[18:29] Question #2

If a person drank a large quantity of hyper-sailing ocean water, the person could die because absorption of salt into the blood will cause it to become:

(A) hypotonic compared with the cytosol of the body’s cells causing osmosis of water into the cells.

(B) hypertonic compared with the cytosol of the body cells causing osmosis of water into the cells.

(D) hypertonic compared with the cytosol of the body cells causing osmosis of water out of the cells.

[19:14] Question Breakdown

The correct answer here is D. A and B are water into the cells. C and D are water out of the cells. So if you drink a lot of hyper-sailing ocean water, you’re going to get a lot of salt in your blood, generically thinking. And so it is hypertonic compared with the body cells. So A and C are out. Because it’s hypertonic, the water in the cells is going to want to go out of the cells to try to balance that out. And so we would end up with D hypertonic compared with the cytosol of the body cells causing osmosis of water out of the cells.

Water is going to move out of the cells into the blood. This is also why drinking salt water is probably not a good idea, like eating salty foods may not be a good idea, but less bad of an idea. Eating salty foods is going to do the same thing. It’s going pull water out of your cells and into your blood, which is gonna cause your blood pressure to go up. That’s why people who have high blood pressure should probably be avoiding salty foods.

[20:43] Question #3

Based on figure one, adding salt to water causes the boiling point of water to:

(A) increase, requiring a greater average kinetic energy of the liquid to produce a vapor pressure equal to the external pressure

(B) increase, requiring a greater average kinetic energy of the liquid to produce a vapor pressure greater than the external pressure

(C) decrease, requiring a lower average kinetic energy of the liquid to produce a vapor pressure equal to the external pressure

(D) decrease, requiring a lower average kinetic energy of the liquid to produce a vapor pressure that is less than the external pressure.

[21:30] Question Breakdown

Adding salt increases the boiling point of water faster. It turns out you have to add a ton of salt to actually make a difference like cut your time from three minutes to two and a half minutes.

So the answer has got to be A or B. This is another kind of pseudo discrete question that requires you to have some outside knowledge that’s a little bit specialized.

The water molecules are bouncing around inside the water. If they’re going the same, they’re hitting the surface of the water with the same pressure as their force of the air pressing down. Then it’s like equal pressure or equal forces, and it’s not going to explode down. So when it hits this, it’s just going to keep going. Because the net force is going to be like zero. Therefore, you want the average kinetic energy of the liquid to be equal, but the vapor pressure has to be basically equal to the external pressure.

And if it’s greater than the external pressure, it would be this superheated fluid that’s going to basically explode into steam. And you don’t want that.

So the correct answer is A, the boiling point of water increases requiring a greater average kinetic energy of the liquid to produce a vapor pressure equal to the external pressure.

[24:02] Question #4

Water is a rare substance in that the solid is less dense than the liquid at the freezing point resulting in a solid form that floats on top of the liquid ice cubes. Which of the following best explains this phenomenon?

(A) The bent structure of the water molecule results in a molecular dipole that maximizes the close molecular packing structure in the solid-state.

(B) The bent structure of the water molecule and ratio of covalently bonded hydrogens to lone pairs of electrons on the oxygen atom maximizes the hydrogen bonding that occurs in the solid phase, producing a hexagonal structure with large empty spaces.

(D) The London dispersion forces of water significantly decrease in the solid phase as compared with the liquid phase.

[25:03] Question Breakdown

The more you know, the better off you are. But this question is asking what explains why ice floats. And B is just saying there’s these big empty spaces. So that makes it less dense.

Whether or not A, C and D are even true, B is the only one that actually explains why something might float. And so B is going to be the best answer there. You can basically get away with this even if you’re unsure of what these things mean overall.

Make sure that you’re answering the question because that’s a common trap that the MCAT sometimes sets. Students are looking through the answer choices which may be true but it doesn’t answer the question. And the question here is what makes the ice float?

[26:58] Question #5

What is the chemical formula for gypsum?

(A) CaSO3

(B) CaSO4

Question Breakdown:

This requires you to know those polyatomic ions, those things that all your teachers have been trying to get you to memorize. The passage itself says that gypsum is calcium sulfate. And so all of these answers have calcium, so that doesn’t help. But only some of them have sulfate.

Here’s a quick way to remember this. There’s like phosphate and phosphite and chlorate and chlorite and all that. The -ate always has more oxygens than the -ite does.

So SO4 is going to be sulfate and SO3 is going to be sulfite. It’s the same for nitrates and nitrites as well. Here’s an easy way to remember that the 881 of the oxygens, right? Yeah. And eight more. It’s bigger.

The key here is knowing that sulfate is not just SO4 but it’s SO₄²-. It’s got a -2 charge. Calcium, if we look at our periodic table, it’s in that second column. That means it’s most likely to lose 2 electrons and so calcium wants to be calcium 2+. So if calcium wants to be 2+ and sulfate is 2-, like the actual proper balanced version of this is going to be Ca.