Osmotic pressure difference,pressure and electrical potential difference.

1. Oct 27, 2012

KloosterG

I have an exam on monday, for a course I'm not familiar with. I have studied the material and tried some example exam question but I can't seem to figure out how to get to the result of the following questions.
Q1:
1. The problem statement, all variables and given/known data
A sample of hydrogel is in equilibrium with a sodium chloride solution of 0.3 moles/l at T=298 K. The fixed charge density of the sample is −0.2 moleq/l.
a. Calculate the osmotic pressure difference between the sample and the solution.
80 kPa
b. Calculate the effective stress in the sample.
80 kPa

3. The attempt at a solution
I tried it by calculating the Donnan-equilibrium concentrations but I have a feeling thats the wrong direction.

Q2:
A cylindrical hydrogel has homogenous, isotropic lineair elastic material properties. The sample is in equilibrium with a sodium chloride solution of concentration 0.15 Mol/l and of pressure zero, and is at temperature T= 300 K. The fixed charge density of the sample is −0.05 moleq/l.
a. What is the pressure in the sample?
10 kPa
b.What is the electrical potential difference between the sample and the solution?
ξ = −4.3mV

a. I started again with calculating the Donnan-equilibrium concentrations, but got stuck after that.
b. No idea where to start.

Thank in advance for anyone willing to help.

2. Oct 28, 2012

KloosterG

Anybody how can help me before my exam tomorrow?

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