How to find the units for Ksp

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1. Dec 15, 2016

Zoey Brown

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. Dec 16, 2016

mjc123

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.