Question

A constant which has no unit

Answer 1

Equilibrium constant K.

Suppose, there is a chemical reaction A + B = C + D

K = ([C][D]) / ([A][B]), where [A], [B], [C] and [D] - equilibrium concentrations of the substances A, B, C and D, measured in mol/L. As everithing is measured in mol/L, then K = (mol²/L²) / (mol²/l²), and so it has no unit, only numerical value.