General Chemistry question

[Kami]

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Can anyone answer the following question:

The osmotic pressure at STP of a solution made from 1L of NaCl(aq) containing 117 g of NaCl is

A. 44.77 atm
B. 48.87 atm
C. 89.54 atm
D. 117 atm

 

[MeowMix]

I look at those answer choices and I instantly think, none of those can be right - 45 atm? that is 45 times atmospheric pressure! No way!

 

[Pinkertinkle]

Osmotic pressure (Pi)

Pi=(Molarity)(GasConstant)(Temperature)

Pi=(2)(.082)(298)

Pi=48

(B)

Too bad I didn't learn that until Physical Chemistry and that's not a MCAT prereq.

 

[Kami]

Hi Pinkertinkle! I got the same answer as you but it appears that the answer is choice C. Actually it is one of the questions from Kaplan text chapter 8 in general chemistry section and they used 4 moles instead of 2 which is why I am trying to find out.

 

[Sunflower189]

Remember, in all the colligative property formulas, it's moles of PARTICLES...not moles of ionic formula.

2 mol NaCl = (2 mol Na+) + (2 mol Cl-) = 4 mol particles

 

[OnMyWayThere]

Plugging in 4 gives me 97.74. That's not reasonably close.

 

[Pinkertinkle]

Hrm, yeah ionic moles sounds right but they don't seem to provide the answer for that.

 

[Sunflower189]

You're confusing your STPs. In this case, you want 0 degrees C = 273K (and 1 atm).

pi = 4m * 0.082 * 273K = 89.54 = answer C

(wow, it didn't all precipitate out of my brain august 17th... )

 

[Pinkertinkle]

Ah and Sunflower saves the day. Good thing i'm not taking the MCAT anymore.

 

[Kami]

Sunflower, this is interesting so that means in colligative properties it is double the amount i.e. instead of 2 moles of NaCl it is going to be two 4 moles NaCl and it is calculated in terms of particles and not ionic.

 

[Sunflower189]

Not necessarily double...it's just the number of particles that something produces in aqueous solution.

1 mol glucose = 1 mol particles
1 mol NaCl = 2 mol particles
1 mol Cu(NO3)2 = 3 mol particles

And so on. Of course, IRL you can have all kinds of other funky stuff, especially in concentrated solutions where ionization is not 100%. But this is test-world, where g=10 m/s2, friction is optional, and things always dissolve on command. 😀

 

[vizzini]

with colligative properties, you have to take into account the VanHoff (spelling?) factor...basically this is all it is: when working with a compound that can break up into multiple ions (like NaCl) you count each one of those separately...so the gas equation looks like this: pi = iMRT so pi = (2)(2)(.0821)(273)

i is VanHoff
M is molarity
R is gas constant
T is temp

you use it for every colligative property, and just for good reinforcement, even if the compound does not break up into charged ions (like glucose), write "1" in your equation...hope this makes sense!

 

[Nutmeg]

Originally posted by Pinkertinkle
Osmotic pressure (Pi)

Pi=(Molarity)(GasConstant)(Temperature)

Pi=(2)(.082)(298)

Pi=48

(B)

Too bad I didn't learn that until Physical Chemistry and that's not a MCAT prereq.

That's why practice tests all suck. Most of it sems irrelevant, and I personally wouldn't waste time with anything other than what is in the aamc tests. All the test-prep companies want to make you think it's a test that can be studied for in the traditional sense by memorizing formulas and crap, when in truth the test is mostly looking at you ability to reason. I'm a senior in chemical engineering and molecular biology, I've taken classes in physical chemistry, mass transport, and thermodynamics (all of which covered osmotic pressure), and I still didn't know the answer to this question when presented, because I don't have those equations memorized. that being said, I still managed a 14 on the PS section.

This question has jack to do with the MCAT, IMO.

 

[Chet Rogers]

Where do you find STP? The question is easy, but we never covered stp in gen chem.

 

[Nutmeg]

Originally posted by Chet Rogers
Where do you find STP? The question is easy, but we never covered stp in gen chem.

STP = Standard Temperature and Pressure, ie 298.15 K, 1 atm. I find it very hard to believe that you never covered it in gen chem, that's honestly like saying they didn't mention DNA in your biology class.

 

[Can U Feel It?]

STP is actually 273 K & 1 atm

http://en.wikipedia.org/wiki/Standard_temperature_and_pressure

 

[Nutmeg]

Originally posted by Can U Feel It?
STP is actually 273 K & 1 atm

http://en.wikipedia.org/wiki/Standard_temperature_and_pressure

Oh, the irony. 😳 I'm used to 25 C because that's the standard condition that is employed in chemical engineering, as it's close to ambient temperature.

 

[docjolly]

Originally posted by Kami
Can anyone answer the following question:

The osmotic pressure at STP of a solution made from 1L of NaCl(aq) containing 117 g of NaCl is

A. 44.77 atm
B. 48.87 atm
C. 89.54 atm
D. 117 atm

Hmmm...
This exact same question is found in the Kaplan Review Notes for General Chemistry...

 

[Kami]

Yes thats right this question is from Kaplans general chemistry chapter 8. By the way how many of you thought that this was an interesting topic. It really has clearified the issue of what is STP and how do colligative properties work 🙂