The equilibrium constant Kc is not dimensionless

The equilibrium constant Kc is a fundamental quantity in solving chemical equilibrium problems.
Many secondary school chemistry textbooks use a dimensionless concentration fraction, which they usually represent with the symbol K. Both the dimensionlessness and the symbol K do not comply with the IUPAC guidelines for a concentration break. We advocate doing away with a dimensionless equilibrium constant in secondary education because IUPAC has defined the obvious equilibrium constant Kc with concentration units. In the so-called Green Book, IUPAC defines several equilibrium constants, such as K , Kp and Kc .
Of these, only the thermodynamic equilibrium constant K is a dimensionless quantity, but the equilibrium constants Kp and Kc are expressed with a unit by the IUPAC definition. We cannot emphasize enough that K and Kc are two completely different equilibrium constants (see figure: the calculated value of both equilibrium constants for the synthesis of ammonia at 298.15 K).