# Why is the equilibrium constant unaffected by a change of the initial concentration?

In my class, it was taught that the equilibrium constant does not vary with the intial concentration of the reactants? Why is it so?

For $$a\mathrm{A}+b\mathrm{B}\rightarrow c\mathrm{C}+d\mathrm{D}$$, $$K_c=\frac{[\mathrm{C}]^c[\mathrm{D}]^d}{[\mathrm{A}]^a[\mathrm{B}]^b}$$ where $$[\mathrm{C}]$$, $$[\mathrm{D}]$$, $$[\mathrm{A}]$$, and $$[\mathrm{B}]$$ are concentrations at equilibrium.
• This is a first order approximation for ideal dilutions only. If the initial concentration becomes large, $K_c\propto K$ breaks down. – Martin - マーチン Sep 27 '18 at 12:56