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Minimum Pressure applied by reverse osmosis?

  1. Feb 28, 2009 #1
    1. The problem statement, all variables and given/known data
    Seawater is approximately .60M NaCl. What is the minimum pressure that must be applied at 25 degrees Celsius to purify seawater by reverse osmosis? Assume i=2.0 for NaCl.

    2. Relevant equations
    Osmotic pressure = MRT
    R= gas constant, .0821 Latm/mol K

    3. The attempt at a solution
    I started by trying to find the osmotic pressure:
    Osmotic pressure = .60M (.0821 Latm/mol K) (298 K)
    = 14.68 Latm

    I feel like this isn't correct because I didn't use the i=2.0 part.
    Can someone tell me what i'm doing wrong?

    should i first calculate the boiling point of the solution using delta t = i Kb(m) ?
    If so, why and how can I calculate that using m?
  2. jcsd
  3. Mar 2, 2009 #2
    The correct formula for osmotic pressure is iMRT when M refers to the solute. So you need to multiple your earlier solution by i=2, giving an answer of 29 atm (don't give too many significant figures as the given NaCl concentration has only 2 significant figures). What i means is that each mol of NaCl when dissolved in water will give 2 mol of ions.

    Hope that helps.
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