Osmotic pressure difference,pressure and electrical potential difference.

[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:

Homework Statement

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

The Attempt at a Solution

I tried it by calculating the Donnan-equilibrium concentrations but I have a feeling that's 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.

 

[KloosterG]

Anybody how can help me before my exam tomorrow?