Osmosis Question

[MDwannabe7]

This is probably a dumb question, but it is stumping me, so...

What happens when you have a container separated into two compartments by a semipermeable membrane, with 1.5M NaCl in the left compartment and 2.0M glucose in the right one?

It seems to me that water would move in the direction towards glucose, since that is both a larger mass and greater molarity. However, apparently water moves from the glucose side to the NaCl side and I can't quite figure out why. I have a note about the effective concentration being 3.0M b/c NaCl dissolves, but that's not clarifying it for me. HELP!

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[cornpops21]

I think it has to do with the fact that NaCl is ionic and can dissociate into 2 particles, so the Vant hoff factor = 2, whereas glucose is an organic compound and n = 1 for it.

 

[engineeredout]

I think it has to do with the fact that NaCl is ionic and can dissociate into 2 particles, so the Vant hoff factor = 2, whereas glucose is an organic compound and n = 1 for it.

Thats it. Remember, osmotic pressure = iMRT. Molarity is a factor yes, but you also have to consider that Va't hoff factor i, which is the number of ions that the compound will dissociate into. If you plug in the numbers, you'll get osmotic pressure for glucose as P = 1*2*RT. For NaCl, you'll get P = 2*1.5*RT, and since 3 > 2, you'll see osmosis from the glucose side to the NaCl side.

Molecular mass has no impact on osmosis.

 

[Vihsadas]

I think it has to do with the fact that NaCl is ionic and can dissociate into 2 particles, so the Vant hoff factor = 2, whereas glucose is an organic compound and n = 1 for it.

Yeah exactly. You have to remember that osmolarity is dependent on the number ions dissolved. So NaCl dissolves into Na and Cl, so there are actually twice the moles of individual ions as the moles of NaCl. Glucose doesn't dissociate so the moles of ions is the same as the moles of Glucose.

 

[MDwannabe7]

Thanks everyone - I get it now!

 

[amar21]

interesting..


I did not know that!

 

[futuredoctor10]

Yeah we had a question about this (I think the exact same one in fact) in our Kaplan Biological Sciences 1 Lecture!!!

 

[hdu]

Vihsadas was correct. Just in case you need a little more information: It happens because Osmotic pressure is one of the colligative properties of solutions, properties that depend on the number of particles and not on the nature of the particles.

Other colligative properties are:

freezing point (decreases while more solutes are in the solution)
boiling point (increases while more solutes are in the solution)
vapor pressure (decreases while more solutes are in the solution)

In all the cases, take in account if the solute is ionic or not, and if ionic, how many ions will be dissolved 2 like in Cl Na solution? or 3 like in Cl2 Mg? or it will not be dissociated like glucose?

I hope it helps

My blog www.biochemistryquestions.wordpress.com

 

[engineeredout]

Vihsadas was correct.

Oh I get it. Vihsadas is just better than everyone else. He is right and I'm wrong. :cry::cry::cry::cry:

 

[hdu]

Sorry, engineeredout,you are very right!

Give the Cesar what belongs to Cesar...

 

[Vihsadas]

Oh I get it. Vihsadas is just better than everyone else. He is right and I'm wrong. :cry::cry::cry::cry:

rofl. I really hope that people don't think that. I do spend a lot of time giving advice, but really hope that people don't take my words as gospel. You've got to find your own stride, and what works best for you personally. There are plenty of amazingly smart people on these boards who deserve a listen (yourself included. )

 

[engineeredout]

Sorry, engineeredout,you are very right!

Man I am such an ass hole