Osmotic Pressure Clarification
I think you've got the solutions the wrong way round and that you should be applying the pressure on the non-pure solution, not the pure solution. Pure water is drawn in through the membrane so you have to put pressure on the non-pure solution to keep it out. Which textbook are you using? See the following quote from Voet et al. 5th ed. p. 30 and figure from Lehninger. Principles of Biochemistry p. 57.


  The osmotic pressure of a solution is the pressure that must be applied to
  the solution to equalize the flow of water across the membrane in both directions
  (the added pressure forces more water from the solution through the membrane);
  it is proportional to the concentration of the solute.