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Consider a desalination process where you enter sea water and receive fresh water and brine (or maybe pure salt).

How do I compute the least amount of energy per mass? I think this has something to do with the entropy of the different mixtures, mostly dependent on the salt concentration in the brine. Is this correct? How to calculate?

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up vote 3 down vote accepted

Assuming that the process is done at constant temperature and pressure, what you want is the Gibbs free energy difference. The reaction could be taken to be 1 mole of water in brine on one side and one mole of pure water on the other side.

The result should be a positive number since the process is not spontaneous. However the value of $\Delta G$ will be the minimum non-mechanical work necessary for the process.

To understand this read up on the Gibbs free energy.

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