ultural soils in California are high in salt content (assume NaCI). Irrigation w

Question

ultural soils in California are high in salt content (assume NaCI). Irrigation water used in growing row crops in CA must be filtered before discharge in rivers and treams, 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 32.59 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 1.0006 g/ml at 20.00 °C)

Explanation / Answer

The required water concentration = 2part per thousand

The actual concentration =35.59ppt

So the concentration to be removed =33.59 ppt

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

Osmotic pressure of given solution = concentration X R X T

Concentration = Moles / L

Moles of NaCl = mass / Molecular weight

= 33.59 / 35.59

= 0.943moles

Volume of solution = Mass / Density

= 1000 grams / Density

= 1000 / 1.0006

= 999.400mL

Molarity = Moles / Litres of solution

=0.943 / 0.999 L

= 0.943 Molar

Osmotic pressure = 0.943 X 0.0821 X 293

= 22.6 atm

So pressure should be greater than 22.6 atm

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