25. DISCUSSION Reverse osmosis is a process for obtaining a pure solvent from a

Question

25. DISCUSSION Reverse osmosis is a process for obtaining a pure solvent from a solution by applying enough excess pressure to the solution to force solvent molecules through a membrane, producing relatively pure solvent on the other side of the membrane and leaving behind a more concentrated solution. The applied pressure must be greater than the osmotic pressure that develops between the solution and the pure solvent. Reverse osmosis is used extensively in arid countries, for example in the Middle East, to generate drinking water from the ocean or other brackish water sources. The method is both expensive and energy intensive, but where fresh water is scarce there are often no cheaper alternatives. ocean water typically contains approximately 35 moles of salt ions dissolved in each 1000 moles of water. Assuming that ocean water has the same density as pure water, use this ratio to determine the molarity of the solution and then calculate and enter below the osmotic pressure between ocean water and pure water at 25 atm

Explanation / Answer

osmotic pressure = MRT

T = 25 C = 25+273 = 298 K

volume = mass of water / density = 1000*18 g / 1000 g/L = 18 L

molarity = 35 / 18

Value of R is 0.082 L atm?K?1?mol?1

Osmotic pressure = (35*298* 0.082)/18 = 47.51 atm

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