Question

Agricultural soils in California are high in salt content (assume NaCl). Irrigation water used in growing row crops in CA must be filtered before discharge in rivers and streams, and is typically performed by using reverse osmosis (RO). The water quality standard in CA for salt in ag runoff water is 2.00 parts per thousand (ppt). If irrigation runoff is 58.08 ppt, what applied (hydrostatic) pressure must be used to filter runoff by RO to meet the CA water quality standard at 20.00 °C? (Note: the density of the NaCl(aq) solution is 1.0024 g/mL at 20.00 °C).

Answer 1

The required water concentration = 2part per thousand

The actual concentration = 58.08 ppt

So the concentration to be removed = 56.08 ppt

this concentration can be removed by applying pressure greater then its osmotic perssure

Osmotic pressure of given solution = concentration X R X T

Conentration = Moles / L

Moles of NaCl = mass / Molecular weight = 56.08 / 58.5 = 0.958 moles

Volume of solution = Mass / Density = 1000 grams / Density = 1000 / 1.0024 = 997.61 mL

Molarity = Moles / Litres of solution = 0.958 / 0.997 L = 0.961 Molar

Osmotic pressure = 0.961 X 0.0821 X 293 = 23.12 atm

So pressure should be greater than 23.12 atm