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 0.227 M in sodium chloride and 0.078 M in magnesium sulfate? Assume complete dissociation for electrolytes.

Answer 1

What we need to know is the molarity of all the particles of NaCl taking into account that NaCl = Na+ + Cl- and the ones of taking into account that MgSO4=Mg+2 + SO4-2. we are going to use a formula to get the molarity like this: M = 2(0.227 M) + 2(0.078 M) 
        M = 0.61 M 
After that to get the atm we do it like this:
Π = (0.61 M)(0.08206 atm-L/mol-K)(25 + 273 K) 
Π = 14.9 atm  
and then we do this multyplication
(14.9 atm)(101.3 kPa/atm) = 1509.37 kPa