PS Timing Issue

[Ariana85]

So, I recently started doing practice tests, and am having some trouble getting thru the PS section quickly enough to finish before the time runs out. I have noticed that on many of the passages, I was able to answer the questions w/out actually reading through the passage, and just referring back to it when needed. Is it better to go straight to the questions, and use the passage as a reference?? I feel like reading through everything is what's slowing me down

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[tncekm]

IMHO there are too many passage based questions to get away with that all of the time. I always like to read through the passage so that I know where to find the answer for the passage based questions. You don't need to read in detail, in fact, I'd say spending more than 1-1.5 mins/passage in PS is generally too much.

Part of the process is just becoming efficient at doing calculations and being confident in your answers.

I.e. Say you have two solutions separated by a semipermeable membrane. One with 0.000021M NaCl and one with 0.0000305M BaF2. What result would you expect?

a) 2NaCl(aq) + BaF2(aq) --> BaCl2(s) + 2NaF(s) and the osmotic pressure between the two sides will equalize as all salts precipitate from the solution.
b) No observable changes will be noted.
c) Water will move from NaCl side to the BaF2 side.
d) Water will move from the BaF2 side to the NaCl side.

So, you'll probably eliminate (a) and (b) right away and be stuck between c and d trying to figure out which side will have the greatest concentration of electrolytes in solution. Rather than working out all of the problem in detail realize that both solutions are of the same "magnitude" of molarity (10^-5) and you can simply ignore all the zeros and multiply the 2 and the 3 by their respective van't hoff factor (2 and 3 respectively) to give:

NaCl --> ∏=iMRT --> ∏=4(RT)
BaF2 --> ∏=iMRT --> ∏=9(RT)

So, 0.000021M NaCl has less parts in solution than 0.0000305M BaF2 so water will move from NaCl side to the BaF2 side which has the greatest osmotic pressure (or "pulling power" as it can inappropriately be mentally modeled).

 

[craniotomy30]

Definitely read the passage first. It is too risky to read the questions first because you may be missing a large piece of info from the passage that is required to answer the question. The passage is helpful because it gives you some background info that may be helpful when answering questions.

 

[Ariana85]

Thanx guys I guess I just need to stop reading the passages so thoroughly and try to skim through it so I know where to find whatever I need to answer each question... Generally, I can get through the calculations pretty quickly, but there's always room for improvement!