General chemistry lays down the foundation for a lot of the sciences on the MCAT. In this episode, Bryan breaks down this general chemistry passage for the new MCAT.
[2:22] Passage #1:
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 (3.5% by mass) and are often associated with flora and fauna that are specifically adapted to extreme conditions. There's considerable recent interest in species that are able to survive under such conditions because they may represent conditions for life that might be present on other worlds.
[3:00] Question #1: What is the approximate molarity of sodium chloride in ocean water if the density of ocean water is 1.028 kg per liter?
- 0.02 molar
- 0.6 molar
- 0.9 molar
- 9.6 molar
[3:20] Bryan's insights:
When there are numbers for answer choices, you should always take a quick look at how spread out they are to know whether you can round them off or you have to be super precise with your calculations.
At first glance, the question seems like it requires 6-8 different calculation steps since you probably have to do certain conversions or go back to the passage. It seems there are at least 2 layers of conversion that have to happen here plus a couple more calculations. But you can take a few short cuts to make this easier to solve.
- Let's say you have a liter of ocean water in front of you. Then you know that liter of ocean water is 1,028 grams. The passage says 3.5% by mass of salt. Take that 1,028 and multiply it by 0.035. Just rounding that off to 1,000, so 1,000 x 3.5% is 35 grams of sodium chloride.
- You have to know the molecular weight of sodium chloride. Bryan will do the work here but on the real test, you will have a periodic table that you can pull out to check the mass of sodium and chloride. But sodium chloride is 58 g/mole. So now you have 35 grams of salt and 35 divided by 58.
But if you don't want to do all that, 35 divided by 70 would be exactly a half. So now you have 35 over about 60. So this is a little more than half or a fraction a little more than half. (As the denominator gets smaller, the number gets a little bit bigger).
[5:57] The Correct Answer
Going back to the answer choices, (a) 0.02, (b) 0.6, (c) 0.9, (d) 9.6. And you arrive at a number that's a little more than half for the number of moles of salt in our hypothetical one liter. So a little more than half is 0.6. Therefore, the right answer is letter b.
So there were only two steps in the calculation. First, taking that 1.028 kg/l and saying that 1,028 grams, 3.5% of that is about 35 grams. Second, convert the grams into moles which is a little more than half a mole. So if you have half a mole in a hypothetical 1 liter, than that's 0.6 molar.
[8:19] Question #2: Based on Fig.1, adding salt to water causes the boiling point of water to __________.
Now we have to decide whether the answer is increase or decrease and what that means in terms of the vapor pressure of the liquid. The question is based on Fig.1 and looking at it, there's a little pressure, temperature, curve, etc., but we don't really need any of that.
The MCAT is going to expect you to walk into the test knowing about colligative properties, which involve changes in boiling point or freezing point. If your background science is real good then you can just answer this straight away.
What happens to the boiling point of water when you add salt in it? It raises the boiling point. If you want to cook stuff quickly, you want your water to be really hot. Throw a bunch of salt in and your water can get hotter.
[10:02] Answer choices:
- increase, requiring a greater average kinetic energy of the liquid to produce a vapor pressure equal to the external pressure
- increase, requiring a greater average kinetic energy of the liquid to produce a vapor pressure greater than the external pressure
[10:20] Bryan's insights:
At this point, that just becomes a definition question. Do you know what boiling point means when it comes to vapor pressures? Again, you've got to walk into the test knowing that by definition, a boiling point is the point where the vapor pressure coming up off the liquid is equal to the atmospheric pressure pushing down on the liquid. Therefore, the answer is letter (a) since boiling point is when those two are in equilibrium with each other.
It's a dynamic equilibrium so the vapor pressure coming up and the atmospheric pressure pushing down are actually equal to each other. However, it doesn't mean the system is static since it's constantly dynamic with the bubbles churning around and the little water molecules popping off into the air.
[11:35] Question #3: What is the chemical formula of gypsum?
Go back to the passage and look what they said about gypsum. The first sentence of the passage says that, “A hyper-sailing body of water contains high concentrations of sodium chloride (salt) and other water-soluble ionic compounds such as calcium sulfate (gypsum).”
You are expected to know the names of ions like sulfate as SO4 as well as the charge of your common cations and anions. SO4 has a charge of -2. Calcium as an alkaline earth metal has a charge of +2. Therefore, this would give you an overall gypsum formula CaSO4.
[12:44] Tricks for Remembering the -ates and the -ites and 3's and 4's:
Hypo- means less of something. Hypo- with an -ite is the least amount of oxygen's. For example, a hyposulfite would just be SO. If you take away the hypo- then it's just the -ite level so there is one more Oxygen. Then -ate is another Oxygen pass that so that gets you up to the SO4 level.
The prefix “per' like in hydrogen peroxide means add an extra Oxygen. So if you have a per- -ate like calcium perchlorate, that would have another Oxygen on it.
Hence, the order they go in would be “hypo-ite,” “-ite,” “-ate,” “per-ate.” Unfortunately, that doesn't tell you the number of oxygen's because it depends on whether it's sulfur or chlorine, or whatever element.
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Dr. Ryan Gray: The MCAT Podcast is part of the Med Ed Media network at www.MedEdMedia.com.
This is The MCAT Podcast, session number 32.
A collaboration between the Medical School Headquarters and Next Step Test Prep, The MCAT Podcast is here to make sure you have the information you need to succeed on your MCAT test day. We all know that the MCAT is one of the biggest hurdles on your premed path, and this podcast will give you the motivation and information that you need to know to help you get the score you deserve so you can one day call yourself a medical student.
Welcome back to, or welcome to if this is your first time joining us, The MCAT Podcast. I am your host, Dr. Ryan Gray, and I host several podcasts on the Med Ed Media network which you can find at www.MedEdMedia.com. Our biggest and most popular show is called The Premed Years where I cover everything that you need to know on your premed path. This week's episode on The Premed Years is an amazing story from a student who was a teen mom, and actually she's a physician now. She was a teen mom, and her path to medical school, and through medical school as a teen mom. You can go check out that episode at www.MedicalSchoolHQ.net/224.
This week we're going to jump into more questions on the MCAT, and we're going to bring back Bryan to talk about them.
Alright Bryan, last week you said biochem was your favorite. Now we're going to step down to just plain old chemistry. How does chemistry rate on your list of favorites?
Bryan Schnedeker: Oh gen chem, general chemistry, absolutely my favorite part of the MCAT. It lays down so many important foundational concepts that just apply to so much in science, that this week general chemistry is my favorite.
Dr. Ryan Gray: Okay so it just depends on the week.
Bryan Schnedeker: Absolutely.
Dr. Ryan Gray: Okay, that makes sense.
General Chemistry Passage Question #1
Bryan Schnedeker: Yeah so we're going to look at a general chemistry passage. As always I very strongly encourage you to head over to the website and print out the show notes handout. For now I'm not going to read the whole thing out because it's a little bit of a longer passage. I'll just read the first paragraph so everybody has a sense of what the passage is talking about, and then we'll dive right into the questions. So the passage starts out, ‘A hypersaline 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 (3.5% body mass) and are often associated with flora and fauna that are specifically adapted to extreme conditions. There's considerable recent interest in species that are able to survive under such conditions because they may represent conditions for life that might be present on other worlds.' And then it goes onto a whole discussion about hyper saline water, so water with lots and lots of ions in it. The very first question then asks, ‘What is the approximate molarity of sodium chloride in ocean water if the density of ocean water is 1.028 kilograms per liter?' And our answer choices are 0.02 molar, 0.6 molar, 0.9 molar, 9.6 molar. And Ryan, we've talked about this before when there's numbers for answer choices, you should always take a quick look at how spread out they are so you know if you can kind of round stuff off a bunch or if you have to be super precise with your calculations.
Dr. Ryan Gray: Yeah.
Bryan Schnedeker: So this- at first glance this question seems like it's going to require a whole ton of probably on the order of six to eight different calculation steps. It says the density is 1.028 kilograms per liter, you have to convert that to molarity, you have to go back to the passage, remember we were reading about ocean water, the passage told us ocean water was 3.5% salt by mass. So it seems like there's at least two layers of conversion that have to happen there, plus another couple of calculations. But we can take a couple of shortcuts to make this a bunch easier. So one thing that we want to think about is just the fact that in one liter- like let's just say we have a liter of ocean water in front of us, just kind of arbitrarily, right? We mention a one liter container. Then we know that the question said it's 1.028 kilograms, so that means that liter of ocean water is 1,028 grams. The passage told us it was 3.5% by mass of salt, so let's just take that 1,028 and multiply it by 0.035 and just rounding that off, I mean the 1,028, let's just round that to 1,000. So 1,000 times 3.5%, it's going to be about 35 grams of sodium chloride. So okay now we've got 35 grams of sodium chloride, and we do have to know the molecular weight of sodium chloride, so there's just one other step here. And I'll do the work for you, but on the real test you would have the periodic table, you could pull that up, check the mass of sodium, the mass of chloride, but sodium chloride is 58 grams per mole, so then you just say, ‘Well I have 35 grams of salt,' and 35 divided by 58, and you go, okay well if I don't want to do all that out, 35 divided by 70, 35 over 70 would be exactly a half. So now I have 35 over about 60. So this is a little more than half, it's a fraction a little more than half, right? If the denominator gets smaller, the number gets a little bigger. So again the answer choices were 0.02, 0.6, 0.9, 9.6. And you arrive at a number that's a little more than half for the number of moles of salt in our hypothetical one liter, so a little more than a half, well that's 0.6 so you get answer choice B there, 0.62 molar. So again really only two steps in the calculation. Taking that 1.028 kilograms per liter and saying that 1,028 grams, 3.5% of that's about 35 grams, converting the grams to moles, a little more than half a mole, and if you have half a mole in a hypothetical one liter, then that's 0.6 molar.
Dr. Ryan Gray: That is a lot easier than what most students I'm assuming are doing, having twenty different steps on their scratch paper.
Bryan Schnedeker: Yeah absolutely, there's this effort to try and convert a density into something else, and then convert that something else into moles, and then- like kind of rearranging the steps, and most students that have done this problem would end up thinking of this as about a four to five step calculation instead of a two step calculation that can be solved by just saying, ‘Well let's assume I have one liter of salt water in front of me, now what?' And the MCAT surprisingly will let you do that a fair bit, that if a question is kind of asked in the hypothetical or the abstract, you just say, ‘Okay well let me imagine I have one liter of this stuff. Or let me imagine I have one gram of this stuff,' and then work from there.
Dr. Ryan Gray: I wonder what students are out there going, ‘Oh man, how many liters of ocean water are there on the plant? Oh man, how can I answer that?'
Bryan Schnedeker: Right, yeah okay radius of the earth, surface area of a sphere is four thirds pi RQ- yeah right. Yeah no, much simpler than that.
Dr. Ryan Gray: Alright so I like the adding in your own hypothetical in there when it allows you to.
General Chemistry Passage Question #2
Bryan Schnedeker: Yeah absolutely. This is actually a strategy that I first learned teaching it to SAT students where you would say, ‘Well just guess that X is ten and work from there,' and it's surprisingly applicable to the MCAT. Almost every time there's an MCAT, there's at least one question where you can go, ‘Well let me just guess that I have this much and work from there.' Okay let's take a look at question number three. ‘Based on figure one, adding salt to water causes the boiling point of water to-‘ and then we have to decide between increase or decrease, and what that means in terms of the vapor pressure of the liquid. Now the question did say according to figure one, and if we go over to figure one we can see a little pressure temperature curve, essentially a phase diagram, solid liquid vapor, and so on. But we really don't need any of that. The MCAT is going to expect you to walk into the test knowing about colligative properties, that's what it's called when the boiling point or freezing point changes. You just have to walk in knowing this. So if your background science is real good here, then you can just answer this straight away without having to do any kind of extensive analysis in the figure. So adding salt to water does what to the boiling point? Ryan do you remember what that does?
Dr. Ryan Gray: Oh man, so I just know from cooking class when you add salt, it raises the boiling point.
Bryan Schnedeker: Absolutely, right and that's one of the things that I always tell students to help remember. If you want to cook stuff quickly, you want your water to be really hot, so throw a bunch of salt in there and then your water can get hotter. That's not actually true in the real world because the amount of salt it would take to [Inaudible 00:09:36] the boiling point, it would be so disgusting you couldn't eat it, it would be way too salty.
Dr. Ryan Gray: I like salt.
Bryan Schnedeker: Yeah right, pour that blood pressure in there. Right. But even if it's not actually practically true it's a good kind of little pneumonic to remember it. Throw the salt in the water to raise the boiling point. And that narrows us down to choices A and B as soon as we know that it's an increase in the boiling point. Then we have to decide between choice A which says, ‘Increase requiring a greater average kinetic energy of the liquid to produce a vapor pressure equal to the external pressure.' Or choice B which says, ‘Increase requiring a greater average kinetic energy of the liquid to produce a vapor pressure greater than the external pressure.' And at this point that just becomes a definition question. Do you know what boiling point means when it comes to vapor pressures? So again you've got to walk into the test knowing this, that by definition the boiling point is the point where the vapor pressure coming up off the liquid is equal to the atmospheric pressure pushing down on the liquid. So if those two are in equilibrium with each other, vapor coming up off the liquid, atmosphere pushing down onto the liquid, then you are at an equilibrium that's a boiling point. So answer choice A there would be the right answer.
Dr. Ryan Gray: Okay I probably would have gotten that one wrong because I'm thinking here, ‘Well the bubbles are kind of exploding so it must be greater than the vapor pressure, the external pressure.'
Bryan Schnedeker: Yeah it would seem like it but it's just one of those classic- I always emphasize the idea of equilibrium in chemistry is a dynamic equilibrium. So the vapor pressure coming up and the atmospheric pressure pushing down are actually equal to each other, but that doesn't mean the system is static, right? It's constantly dynamic with the bubbles churning around, and the little water molecules popping off into the air and so on.
Dr. Ryan Gray: Okay.
General Chemistry Passage Question #3
Bryan Schnedeker: And then let's wrap up with a real short one here. What is the chemical formula of gypsum? Okay no reason you should know that walking into the test, so obviously we've got to go back to the passage and look up what they said about gypsum, and that was in the first couple sentences here, so I'll just re-read the first sentence of the passage. ‘A hyper saline body of water contains high concentrations of sodium chloride salt, and other water soluble ionic compounds such as calcium sulfate gypsum.' So you are expected to know the names of ions and such. So sulfate you should recognize as SO4, and you are expected to walk into the test knowing the charge on your common cations and anions. So sulfate, SO4, very common cation, sulfuric acid and so on has a charge of minus two, and calcium as an alkaline earth metal would in its standard state found in nature have a charge of plus two. So a calcium with a plus two, a sulfate with a minus two would give us an overall gypsum formula of calcium SO4. And so that's answer choice B.
Dr. Ryan Gray: Do you have any tricks for remembering the ate's and the ite's and three's and four's and all those other-?
Bryan Schnedeker: You do have to remember the general trend, right? Which is that the- okay so we know the prefix hypo means less of something. So hypo blah, blah, blah ite with an I is the least amount of oxygens. So like a hyposulfite would just be SO, just a single O. Then if you take away the hypo, you get up to just the ite level, so one more oxygen if you have an ite. Then ate is another oxygen past that, so that gets you up to the SO4 level. And then we also want to remember the prefix per, like in hydrogen peroxide, the per means add an extra oxygen. So if you have a per- ate, so some sort of calcium perchlorate or something, then that would have another oxygen on it. So the gradations hypo ite, ite, ate, per ate is the order that they go in. Unfortunately that doesn't actually tell you the number of oxygens because it depends whether it's sulfur, or chlorine, or nitrogen, or whatever, which is a whole lot of me talking Ryan, to just give you the answer no. No I don't have a real simple answer to that.
Dr. Ryan Gray: Alright well we'll have to figure one out for the student listening to make the MCAT that much easier. I'm sure there's some brain out there that has figured this out. So if you're listening to this and you have something that will help, let me know. Ryan@medicalschoolhq.net.
Alright there you have it. I hope that Bryan did a fantastic job breaking down that passage. If you've been looking for the handouts that go with these episodes, I had some issues uploading them before, but if you go to www.TheMCATPodcast.com/32 for this episode, there will be a link in that blog post, the show notes there, that says ‘Download the handout here,' or whatever that link says. Just click that link, put in your email address, and the show notes will be emailed to you right away. So if you've been looking for those, I apologize, all of the old handouts have been fixed and are on all of the episode pages now. So if you are looking for session 30 for example, go to www.TheMCATPodcast.com/30 and they'll be there.
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