1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor

    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted