Water purification by reverse osmosis

The process of reverse osmosis is used to purify water because it removes many contaminants at relatively low cost. The osmosis process can be reversed by applying a pressure greater than the osmotic pressure to an aqueous solution. This causes the water, but not the dissolved solutes, to flow from the solution through the semipermeable membrane. The largest municipal desalination plant in the United States is in Sarasota, Florida. It treats salt water by reverse osmosis to produce more than 12 million gallons of fresh water per day. Other applications include the desalination of sea water on ocean-going vessels and wastewater purification. 

Colligative Properties Compare the number of particles produced per formula unit for these three solutes. Colligative properties depend on the total concentration of particles. 

Remember that coUigative properties depend on the total concentration of solute particles regardless of their identity. The changes in coUigative properties caused by electrolytes will be proportional to the total molality of all dissolved particles, not to formula units. For the same molal concentrations of sucrose and sodium chloride, you would expect the effect on coUigative properties to be twice as large for sodium chloride as for sucrose. What about barium nitrate, Ba(N03)2 Each mole of barium nitrate yields 3 mol of ions in solution. 

You would expect a Ba(NOg)2 solution of a given molaUty to lower the freezing point of its solvent three times as much as a nonelectrolytic solution of the same molality. 

Sample Problem F what is the expected change in the freezing point of water in a solution of 62.5 g of harium nitrate, Ba(N03)2, in 1.00 kg of water  

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Kaufman, Y., A. Berman, and V. Freger, Supported Lipid Bilayer Membranes for Water Purification by Reverse Osmosis, Langmuir, 25, 2010 (pages 7388 - 7395). 

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