How to Break Down a General Chemistry MCAT Passage

Session 32

General chemistry lays the foundation for a lot of the sciences on the MCAT. In this episode, we break down a general chemistry passage for the MCAT.

[2:22] The General Chemistry MCAT Passage

Here is the sample MCAT general chemistry passage we will be taking a look at:

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 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

Question: 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

Molarity is the concentration in moles per literClick To Tweet

[3:20] Breaking Down the Question

When there are numbers for answer choices, you should always take a quick look at how spread out they are to know if you can round or if 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 conversions. 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 shortcuts to make this easier to solve.

Let’s say you have a liter of ocean water in front of you. You know that liter of ocean water is 1,028 grams. The passage says it’s 3.5% salt by mass. So take that 1,028 grams and multiply it by 0.035. We can round the 1,028 grams to 1,000, so 1,000 x 3.5% is 35 grams of sodium chloride. So you have about 35 grams of sodium chloride in that liter of ocean water.

You have to know the molecular weight of sodium chloride. 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/mol. So now you have 35 grams of salt. If you divide 35 grams by 58 g/mol, you’ll get the number of moles of salt in that liter.

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 58, which is about 60. So this is a little more than half of a mole of salt in that liter of ocean water. (As the denominator in a division problem gets smaller, the answer 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 of that 1,028 grams, 3.5% of that is about 35 grams. Second, we converted the grams into moles of salt, and it came out to a little more than half a mole. So if you have half a mole in a hypothetical 1 liter, then that’s 0.6 molar. (Molarity is the concentration in moles per liter.)

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[8:19] Question #2

Question: Based on Fig.1, adding salt to water causes the boiling point of water to __________.

Know Your Colligative Properties

The question is based on Fig.1, which has a little pressure/temperature curve, 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 in science is really good, then you can just answer this straight away.

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.Click To Tweet

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. So we have these two answer choices to select from:

[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] The Answer

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 at which 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, that doesn’t mean the system is static. It’s constantly dynamic with the bubbles churning around and the little water molecules popping off into the air.

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[11:35] Question #3

Question: What is the chemical formula of gypsum?

Breaking Down the Question

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 (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 of CaSO4.

[12:44] Tricks for Remembering Common Ions (the -ates and -ites)

Hypo- means less of something. Hypo- with an -ite is the least amount of oxygens. 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 past that, so that gets you up to the SO4 level.

The prefix “per’ like in hydrogen peroxide means to add an extra Oxygen. So if you have a per- -ate like calcium perchlorate, that would have another Oxygen on it.

So, the order they go in, from least to most oxygens, is “hypo-ite,” “-ite,” “-ate,” “per-ate.” Unfortunately, that doesn’t tell you the precise number of oxygens because that varies depending on whether it’s sulfur, chlorine, or whatever other element.

Remember your ions: The order they go in, from least to most oxygens, is 'hypo-ite,' '-ite,' '-ate,' 'per-ate.'Click To Tweet

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