Question

Calculate the minimum external pressure that must be applied in a reverse osmosis system to reduce the salinity of a seawater sample from 3.00% NaCl (i = 1.84) to 175 mg NaCl per liter at 25°C. Assume the density of the seawater sample is 1.0 g/mL.

Answer 1

The minimum external pressure required can be calculated by the below formula

P = iMRT -----> 1
where
P = Osmotic pressure
i = Van't hoff factor = No. of ions 1 molecule of solute will dissociates into = 1.84
M = molarity (mol/L) = 175 mg / L = (0.175 g/58.44 g/mol) /L = 0.003 mol/L
R = ideal gas constant = 0.08206 L.atm/mol.K
T = 25°C = 298K
Substituting the above values in equation 1
P = 1.84 x (0.003 mol/L) x (0.08206 L.atm/mol.K) x (298K) = 0.135 atm

Minimum external pressure that must be applied in a reverse osmosis system to reduce the salinity of a seawater= 0.135 atm