[meghan]

Hi!

I don't understand the very last step in this problem where the solution for osmotic pressure is multiplied by 2.  Is this because there are two particles for the NaCl molecule?  See below.

Question: Calculate the osmotic pressure (atm) expected for a soln of 5.00g NaCl in 350.0g of an aqueous soln at 50.0^oC. 

Answer:

M=.0855 mol NaCl/.350 L solv = 0.244 mol/L

Osmotic pressure = MRT
= .244 mol/L x (0.0821 L x atm/mol x K) x 323K
= 6.47 atm

Osmotic pressure = 6.47 atm x 2 = 12.9 atm.     This is the step I don't understand. 

Any guidance is much appreciated!  Thanks. 

[Borek]

Van 't Hoff factor.

[meghan]

Thanks for the fast response, I'm not sure I understand.  Based on your reply about the Van't Hoff factor, I think the answer would be assuming complete dissociation (2 particles in NaCl), right?  I think the Van't Hoff factor for this equation is a different number. 


Here is a similar question just to make sure I'm understanding correctly. 

I solved for the osmotic pressure of a 60g glucose (C6H1206) soln in 1L of water at 310K.  I found osmotic pressure to be 8.47 atm after multiplying MRT.  Now, do I multiply 8.47atm by 24 (the number of glucose particles in the solution) for the final answer?

Thanks!

[sjb]

Does glucose ionise in aqueous solution?

[meghan]

Thanks for the response!  Just to be sure I'm okay with this now, here is what I'm understanding. 

When working with these types of equations, I have to consider if the solute will ionize in the solvent.  If it will, then I need to consider the particles in the final answer based on complete dissociation or Van't Hoff factor, right?

[Schrödinger]

Yup, that's right.

You might want to read this http://en.wikipedia.org/wiki/Van%27t_Hoff_Factor