#K_"eq"# is the so-called thermodynamic equilibrium constant....and thus for the reaction....
#A+Brightleftharpoons C+D#
#K_"eq"=([C][D])/([A][B])#....#K_"eq"# is properly dimensionless....
I grant that not all equilibria are set up like this, and conceivably we COULD have....
#A+BrightleftharpoonsC#
...the which, ostensibly, could have units. In these scenarios we use DIMENSIONLESS activities to report the concentration of #[A]#, #[B]#, and #[C]#.
And why do we make all these specifications? Well, because of the #"Gibbs Free Energy equation...."#, the which specifies that....
#lnK_"eq"=-(DeltaG^@)/(RT)#....and establishes the relationship of the thermodynamic equilibrium constant to the free energy, and of course we may only take #"logs"# of a dimensionless number....
