Question

What is the minimum pressure in kPa that must be applied at 25 °C to obtain pure water by reverse osmosis from water that is 0.204 M in sodium chloride and 0.022 M in zinc sulfate? Assume complete dissociation for electrolytes.

Answer 1

T = 25 + 273 = 298 K

for NaCl (i) = 2

for ZnSO4 (i) = 2

osmotic pressure (P) = [(i C)NaCl + ( i C)ZnSO4 ] R T

                                  = [2 x 0.204 + 2 x 0.022] x 0.0821 x 298

                                   = 11.06 atm

1 atm = 101.325 kPa

P = 11.06 x 101.325 kPa

   = 1120.5 kPa

minimum pressure needed to get reverse osmosis = 1120.5 kPa