Homework Help: Osmotic pressure help

1. Oct 1, 2014

thercias

1. The problem statement, all variables and given/known data t
You are trying to make artifiial blood cells. You have managed to get pure lipid bilayers to form spherical bags of radius 10um, filled with hemoglobin. The first time you did this, you transferred the "cells" into pure water and they promptly burst, spilling the contents. Eventually you found that transferring them into 1 mM (one mili molarity) of salt solution prevents bursting, leaving the "cells" spherical and full of hemoglobin and water.
a) If 1 mM is good, then would 2 mM be twice as good? What happens when you try this?
b) Later you decide that you don't want salt outside because it makes your solution electrically conducting. How many moles per liter of glucose should you use instead?

2. Relevant equations
p = cK(b)T
c = n/V
n = m/molar mass

3. The attempt at a solution
So for A, since 1 mM made the solution isotonic, making the concentration of the solution higher would make the water from the cells go into the solution, shrivelling the cells. So if you increase to 2 mM, then it would not be a good idea.

For B, I'm kind of confused. the Van't hoff relation formula is p(equil) = cK(b)T. This pressure is necessary for keeping the integrity of the cell. So when c = 1mM for the saline solution, it was able to achieve this integrity.
p = 1mM*c*k(b)T

So wouldn't this also work for glucose, keeping the concentration c =1 mM concentration making the pressure the same? I feel like I'm missing something.

2. Oct 2, 2014

Staff: Mentor

Actually you can't solve the problem without knowing the identity of the salt (I suppose you are expected to assume NaCl, but it doesn't have to be correct).

I am not sure you use the correct formula for the osmotic pressure (or at least I don't I understand meaning of the symbols). Please check the formula and name the symbols.

Van 't Hoff is definitely a name that is important here.

3. Oct 2, 2014

thercias

yes, they are talking about sodium chloride. And what do you mean about the correct formula? I'm using Osmotic pressure = concentration(number density of solute molecules) * boltzmann constant * temperature. What formula am i supposed to use?

4. Oct 2, 2014

Staff: Mentor

OK, I am used to the version with gas constant of molar concentration, but these are equivalent. You put Boltzmann constant once as K(b) and once as k(b), while it is typically marked as kB, and you used c for concentration (not explaining what kind of concentration it is, so I assumed it is the most popular molarity), don't be surprised I couldn't understand what you mean.

Concentration of "solute molecules" and "concentration of salt" are two different things. Google for the "Van 't Hoff factor".

5. Oct 2, 2014

thercias

Ok, so I googled it. the van't hoff factor of glucose is 1. the van't hoff factor of nacl is 2.
does this mean glucose will have half the concentration as nacl?

edit:

osmotic pressure = (van hoff factor) * c * boltzmann constant * temperature
for glucose, if the van't hoff factor is half of the salt van't hoff, then the concentration would need to be twice as much as nacl for it to equalize. is this right?

Last edited: Oct 2, 2014
6. Oct 2, 2014

Yes.