32: How to Break Down a General Chemistry Passage

Session 32 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? Answer choices: 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? Bryan's Insights: 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. Links and Other Resources: Download the handout here. Next Step Test Prep (Use the code MCATPOD to save some money on their offerings) MedEd Media Network Get the Handout Click Here to Download Handout

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