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 0.217 M in sodium chloride and 0.012 M in magnesium sulfate? Assume complete dissociation for electrolytes.

Answer 1

Ans: For the reverse osmosis the pressure must be greater than the osmotic pressure of the solution.

The minimum pressure will be equal to the osmotic pressure of the solution.

Osmotic pressure, π = CRT            where, C = concentration of the solution

NaCl ---> Na⁺ + Cl^-

MgSO₄ ---> Mg²⁺ + SO₄^2-

As both salt dissociates 100% in the solution final concentration of the solution after dissociation of the salts is

=( 0.217 + 0.217 + 0.012 + 0.012 ) M

= 0.458 M

Minimum pressure

= 0.458 moles.lit^-1 * 0.082 lit.atm.mol^-1.K^-1 * 298 K

= 11.19 atm

= (11.19 * 101.325) kPa

= 1134 kPa