AAMC 7: SPECIFIC PHYSICAL Q's

[Fups]

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okay. I took AAMC 7 yesterday, and figured out many of my stupid mistakes, BUT there are a few questions on the exam, where i dont seem to be able to do the calculations confidently-- and they are easy...

can anyone answer/explain these questions/answers for me?

#54: What is the molality of a saturated solution of Pb(NO3)2 (aq) at 0 degrees celcius?

the info they give you is: formula weight for Pb(NO3)2 is: 331 g/mol
solubility at 0 deg C: 37.7g / 100 mL

the answer is 1.14 m

#60: same passage: what is the mole fraction of lead nitrate in 10% (wt-wt) aqueous soln?

you can use the same information from above...

how do you incorporate the 10% into the calcualtions?

Thanks guys!
I appreciate any help you can offer!

 

[tigress]

Okay, so for the first one: You know the solubility is 37.7 g/100 mL and the formula weight is 331 g/mol. Molality is moles of solute per kilogram solvent. So you want to find out how many moles are in 37.7 g of the lead nitrate, and then you know moles per mL. You also know that there are 1000 mL per kg of H2O. So to find moles of lead nitrate per kg H20, you do:

37.7 g/100 mL x 1 mol/331 g x 1000 mL/1 kg H2O = 377/331 mol

That's a little over one, so it's got to be 1.14 m.

Now the second one. I was short on time so I skipped this one when I took the test. Here's the answer (courtesy TPR): When it says 10% (wt-wt), that means the lead nitrate is 10% of the weight of the entire solution. You can just pretend you have 100 g of solution to do the problem, and in that case you have 10 g of lead nitrate per 100 g of solution, so 10 g lead nitrate and 90 g water. Find out how many moles are in 10 g of lead nitrate and how many moles are in 90 grams of water. Then divide the moles of lead nitrate by the total solution (moles water plus moles lead nitrate).

10 g Pb(NO3)2 x 1 mol/ 331 g = about 0.03 mol

90 g H2O x 1 mol/18 g = 5 mol

0.03/(5 + 0.03) = about the same as 0.03/5 which is 0.06/10 = 0.006

 

[Fups]

Thank you so much, tigress!
your explanations literally walked me through each calculation, which was exactly what i needed... #54 wasnt so bad... #60 made sense after you explained it... not only did i not know was 10% wt-wt meant, but i had no idea how to approach the problem in an effective, systemmatic way.... now, it is just a matter of doing these calculations FASTER so i don't lose time on them...

now, i have one more question that i didnt ask previously b/c it relies heavily on the depiction...

#39: Passage VII: "To keep the current constant during the discharge cycle:"

the answer is B: the resistance R must be continually decreased.

i actually thought it wold be the opposite.

i know this question relies on V=IR, but... 😕

 

[firebird69guy]

same passage, #59..

I guess I got thrown off because he equilibrated PbN03 in 100ml water for "SEVERAL DAYS." Therefore, I thought that this would be the way you would prepare a supersaturated solution. Apparently not.

So.. how would one go about forming a supersaturated solution?


Thanks!

 

[tigress]

Thank you so much, tigress!
your explanations literally walked me through each calculation, which was exactly what i needed... #54 wasnt so bad... #60 made sense after you explained it... not only did i not know was 10% wt-wt meant, but i had no idea how to approach the problem in an effective, systemmatic way.... now, it is just a matter of doing these calculations FASTER so i don't lose time on them...

now, i have one more question that i didnt ask previously b/c it relies heavily on the depiction...

#39: Passage VII: "To keep the current constant during the discharge cycle:"

the answer is B: the resistance R must be continually decreased.

i actually thought it wold be the opposite.

i know this question relies on V=IR, but... 😕

First off, while I did get the first one right, really I'm mostly using the TPR explanations on here, so thank TPR, not me! 🙂

Okay, so for #39: The discharge cycle will happen after the capacitor is charged and the switch is closed to the right. You know that for a capacitor V is proportional to Q, so as the capacitor discharges and Q decreases, V also decreases. And since V=IR, or I=V/R, if V decreases and you want to keep I constant, you have to decrease R as well. If you increased R while V decreased, I would get smaller and smaller.

 

[Fups]

right!

i feel like a jack@#$ asking this, but what does "TPR" stand for?

 

[tigress]

same passage, #59..

I guess I got thrown off because he equilibrated PbN03 in 100ml water for "SEVERAL DAYS." Therefore, I thought that this would be the way you would prepare a supersaturated solution. Apparently not.

So.. how would one go about forming a supersaturated solution?


Thanks!

Well the thing is, he equilibrated it for 3 days but at 0 degrees C, so the most that could possibly go into solution is still however much would go in at 0 degrees. I found this page that shows some graphs that explain a supersaturated solution http://www.chemcool.com/regents/physicalbehaviorofmatter/aim4.htm
The way you make a supersaturated solution is to heat up the sollvent, add as much solute as will dissolve at that temperature, and then slowly cool the solution. By doing that you will get more solute in solution than you should be able to at that temperature. Have you ever made sugar syrup or candy on the stove? You heat up water to boiling and put as much sugar as will dissolve, then a bunch of the water boils off, and the solution becomes even more concentrated. When you cool it slowly it is very super-saturated with sugar.

So just leaving the thing there at such a low temperature for a length of time wouldn't make a supersaturated solution. On top of that, supersaturated solutions are very unstable, so the solute will precipitate out if left too long.

 

[tigress]

right!

i feel like a jack@#$ asking this, but what does "TPR" stand for?

oh sorry, its The Princeton Review
don't feel silly for asking!

 

[Shrike]

i feel like a jack@#$ asking this, but what does "TPR" stand for?

As she said, don't worry. The presence of the T doesn't make sense to me either, and I've worked for TPR for quite a while. Seems a bit pretentious to me, but The Princeton Review is the company name, and to those in the know it is always TPR.

 

[Fups]

bizarre, but... whatever!

here goes AAMC 8!