Question

Why is a thermodynamic equilibrium constant unitless?

Answer 1

Consider the following reaction.

A+B C

The Keq is as follows.

Keq =    [C]eq/ [C] ss

  [Aeq/ [A] ss X [B]eq/ [B] ss

Here [x] eq standsfor the concentration of the species in the reaction and [x] ss refers to the concentration of a defined standard state. Therefore, the k eq is only a ratio.

You are most probably confused because most if not all text books ignore the [X]ss values in the calculations. This confusion can be resolved by using activity values which are dimensionless rather than concentrtion values which have dimensions.