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How do I determine volume of salt water mixture before mixing?

  1. May 20, 2008 #1
    I read that when salt and water are mixed that the overall volume decreases by 2.5% (I'm not sure if this is relative to the original volume of the water or both the water and the salt). I'm not sure why this is the case and am interested in that.

    Moreover, I'd like to know how one would calculate the final volume of such a mixture. I know one person who believed if a gallon of salt was dumped into 100 gallons of water that the final volume would be 101 gallons. I also know a different person who believed the final volume would be 100.

    Are there straightforward ways to calculate things like this? I remember doing calculations with mols in college but I'm not sure if we ever worked on mixture volumes.


  2. jcsd
  3. May 20, 2008 #2
    Volumes don't add. If you had a barrel of BBs and a barrel of baseballs would you get two barrels full of stuff if you poured the BBs into the baseballs? Of course not since the BBs could fit in between the baseballs.

    I'd expect the volume of the salt water to be less than 100 gallons since the water molecules are going to solvate sodium and chloride ions (they will simply be held closer together). As for a straight forward calculation-- none that I know of off the top of my head.
  4. May 21, 2008 #3


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    I remember a lab in college where we had to investigate such concepts with ethanol and water. You are going to want to research the topic of partial molar volume.

  5. May 21, 2008 #4


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    The only reasonable practical approach is to use density tables. Each solution is different, density changes are different and they are quite often non-linear.

    Simplest approach to calculation of the final volume is to find % w/w concentration (this is volume independent), check density tables to see what density this solution have, then to calculate volume from known total mass and density.
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