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How to find the units for Ksp

  1. Dec 15, 2016 #1
    1. The problem statement, all variables and given/known data
    I have found, through experimentation, the ksp of calcium hydroxide to be 3.0 x10^-7 (which i know has a pretty large percent error). However, i am conflicted as to what the units should be because logically, plugging the units into the ksp equation should give you units of M^3 (ksp= [ca][oh-]^2. However, after research online, i found that ksp doesnt have units. Why does Ksp have no units if it is seemingly so easy to calulate it?

    2. Relevant equations


    3. The attempt at a solution
    One idea i was thinking about was that the ksp units are unique to each equation which may be why they are omitted?
     
  2. jcsd
  3. Dec 16, 2016 #2
    The units for the solubility product should indeed be M3. I have not found that "Ksp doesn't have units" online. However, insofar as a solubility product is an equilibrium constant, strictly speaking equilibrium constants should be dimensionless, as ΔGo = -RTlnK, and the argument of a logarithm must be dimensionless. This is achieved by dividing each concentration by the concentration of the standard state, i.e. 1M. So
    Ksp = ([Ca2+]/1M)([OH-]/1M)2
    which has numerically the same value as your Ksp.
    However, I like to keep the units, as it's a useful check on whether you're getting the powers right.
     
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