Agricultural soils in California are high in salt content (assume NaCl). Irrigat

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 64.32 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 1.0028 g/mL at 20.00 °C).

Explanation / Answer

The applied pressure in Reverse osomosis is pressure to overcome the osmotic pressure generated due to presence of salts in the water.

Osmotic pressure = concentration of solute X R X T

CA standards = 2ppt

the part of salt in water to removed = 62.32 part / thousand = 62.32 grams / 1000grams of solution

which means we have 64.32 grams in NaCl so moles = Mass / Molecular weight = 62.32 / 58.5 = 1.065 moles

Volume = Mass of solution / Density of solution = 1000 / 1.0028 = 997.21 mL

so molarity = Moles of solute / Litres of solution = 1.065 / 0.997 = 1.068 Molar

Osmotic pressure = 1.068 X 0.0821 X 293 K = 26.69 atm

So the pressure required will be greater than 26.69 atm

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