# Does it matter what initial concentration of reactants and products are used to determine the equilibrium constant?

Jul 20, 2017

No, because the equilibrium constant is found from EQUILIBRIUM concentrations.

Whenever you use initial concentrations, you can only locate where you are in the progress of the reaction, i.e. what the reaction quotient $Q$ is.

For

$a A + b B r i g h t \le f t h a r p \infty n s c C + \mathrm{dD}$,

$Q = \frac{{\left[C\right]}_{i}^{c} {\left[D\right]}_{i}^{d}}{{\left[A\right]}_{i}^{a} {\left[B\right]}_{i}^{b}}$,

where all concentrations are initial (or current, if you will).

The equilibrium constant $K$ is defined in the same way, except with ${\left[\text{ }\right]}_{e q}$. The only difference is their values, and the way the numbers are plugged in is the same.

Using an ICE Table...

• one would be able to start from initial concentrations, already knowing the equilibrium constant, to find the change ($x$) in concentrations, regardless of the initial concentrations chosen (because $x$ is dependent on what $K$ is).
• one would be able to start from initial concentrations, already knowing the change in concentrations ($x$), to find the equilibrium constant $K$ (provided the change in concentrations is correct!), because $K$ does not change at the same temperature for the same reaction.