Now here, of course K_"eq"=([A][B])/([C][D]), which of course is unitless because the mol*L^-1 cancels out. But of course not all equilibria are set up like that, we could conceive of, A+BrightleftharpoonsC, where the units would not cancel out.
Strict application of thermodynamic principles relies on activities, a, not concentrations, where the activity is dimensionless by reference to a standard concentration, i.e. a_A=[[A]]/[[A_"standard"]].
Thus when we use the Nernst equation DeltaG^@=-RTln(K_"eq"), we can properly take the logarithm of a dimensionless quantity.
So in summary, we should use dimensionless "activities", however, the A_0 values should be near as dammit to the molar concentration.
