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Official Prep Chemistry Question Pack

Posted: Sat May 23, 2020 12:30 pm
by kleshchuk
12. I don't understand how they were able to do their estimations.
14. I understand that when electron go from lower energy orbitals to higher energy orbitals they absorb energy. I am confused how that causes nickel-ions to be green.

21. I was able to get rid of PNP and Mg2+. It tells us in the beginning of the passage that those are catalysts as a result they cannot be consumed. Meaning they will not affect the yield of polymer. I am confused with there reasoning why water can't affect the yield of polymer. It says the affect would decrease the concentration of reactants. Thus, slowing down the rate of the reaction.

Also, in there explanation they say that in experiment 2 the stress was to remove product by precipitation. This would cause the system to increase product to counteract that action. I understand the basics of LeChatelier's Principle, but I am confused how it works with the manganese sulfate. How does it actually remove more product?

23. I am confused how they got the relationship between concentration of polymer and hydrogen phosphate.
30. When looking at the ionization energy trend, I thought that it would not affect noble gases because they have a full octet.
40. Why is hydrogen considered a nonmetal?
48. I understand the assumptions that are made with the ideal gas law. Unless I misunderstood, there seems to be a connection made that the more stable the molecule is the more the molecules deviate from the ideal gas law. Also, it says room temperature. The standard temperature based on the ideal gas law is 0 degrees Celsius.
50. In a hydrolysis reaction, water is added to break up molecules. How is no2- reacting with water yielding HNO2 and oh- a hydrolysis reaction? I understand that if you add more OH- in a solution it will become more basic. I also understand that NO2- will pick up a hydrogen atom. I just don't understand why they used the term hydrolyze, when it is not a hydrolysis reaction.
52. I was able to cross out molecular weight and solubility in water. I ended up choosing dipole moment, but I just wanted to know if there is any relationship between dielectric constants and dipole moments that I should be aware of.

Re: Official Prep Chemistry Question Pack

Posted: Fri May 29, 2020 2:44 am
by NS_Tutor_Mathias
From a few threads down:
12.
Assuming you are fine with the grams to mol conversion (simply g/molar mass).

Step 1: Eyeball proportions. About the same moles of each. Cool, one estimate will suffice.
Step 2: Eyeball quantities. 262 is about 250. 6.57 is about 650*10^-2.
Step 3: Do magic. 250 * 2.5 = 625. Close enough. Therefore, 250 * 2.5 * 10^-2 (effectively 'divide by 100') = 650*10^-2
Step 4: Translate to English: 250 goes into 6.25 about 2.5/100 times, or 0.025 times.

This might be a little hairy to try at first, but think about this a while longer: If I get unpleasant numbers to work with, or a division into less that will yield ugly fractions, I can just multiply the number I want to divide until it is big enough to give a nice estimate. So rather than 6.56/262.84 I did 656/262.84 and then divided the result by 100.


23.
This is possibly my favorite question in the entire QPack. It is simple but very clever.

Each time a unit is added to the polymer, another HPO4^2- is added to solution (with the exception of the first addition being a very slight irregularity). If we don't think this through, we would now expect the concentration of polymer to rise in conjunction with the phosphate ion concentration. But polymers are by nature polymeric, meaning several units (mers) forming a single molecule. A single molecule only contributes once to it's concentration in solution - so as we proceed, the concentration of polymer MUST be decreasing. In fact, the concentration of the polymer decreases in proportion to how long we've made this polymer. If we were to create a bunch of CpC polymers (n=1) then the concentration of these would be equal to the HPO4^2- concentration. If we were to however create a bunch of CpCpC polymers (n=2), then the concentration of our polymer would be 1/2 times the HPO4^2- concentration. And so on for ever-longer polymers.

This is possibly a little infuriating to deal with, but also a very satisfying question in the sense that it has you think about how polymerization naturally has an effect on the solution environment and interacts oddly with the very concept of solubility.

24. No, the math provided in this unusually extensive AAMC solution is totally correct. A buffer system is just one containing a significant quantity of a weak acid or base and it's conjugate acid/base that thereby doesn't change pH all that quickly in response to changes, such as the addition of hydroxide or hydronium ion. Naturally, in order to be a buffer system, it must be able to operate within a relatively wide range of concentrations - in fact, we are trading a change in hydroxide or hydronium ion concentrations in solution for a change in weak conjugate acid/base.
#14:
The answer key here simply states that non-green wavelengths of the visible light spectrum are being absorbed, so what remains to be reflected is green. Hence the solution appears green. This is largely correct, but iirc it may also have a wee bit to do with the hexahydrate that it forms in solution. You are not expected to somehow know this - but simply that neither A, B nor D are viable answer choices (resonance stabilization of sulfate explains why it is a common anion, but not why it would have any particular color, for instance).

#30:
Why would noble gases be an exception? They're unreactive and are an exception to the electron affinity pattern, but not the ionization energy pattern. After all, as the answer states, first ionization energy is the amount of energy required to remove the first valence electron. This is higher for Krypton than for it's neighbor Bromine, because of the higher effective nuclear charge felt by each electron.

#40:
Because it reacts differently. It ionizes less easily, for starters. Don't worry about this much besides that it is. Classifications are assigned by property, rather than strictly by location in the periodic table.

#46:
If you are referring to answer choice D, at room temperature, molecular volumes and intermolecular forces are certainly not significant. However, at ludicrously high pressures, they most certainly are significant. Therefore, at an extremely high temperature, we expect significant deviations from the ideal gas law. The discussion of stability is only to have you realize how far in excess this pressure is, and that being in the solid phase is clearly a deviation from the ideal >gas< law.

#50:
You've kind of answered this yourself. Water is reacting with NaNO2 and forming Na+, OH- and HNO2. It is only a bit unusual to think about, as most biological hydrolysis reactions feature larger molecules being broken apart - and we don't usually figure the dissolution of a basic salt like this as a hydrolysis reaction. But best I can tell it fills all the criteria. Tricky!

#52:
Better insulators have a lower dielectric constant. Conversely, charged species in solution improve that solutions ability to allow charges to travel - meaning they also raise it's dielectric constant. This is why we can for instance measure the pH with a simple electric potentiometer.