# How to find equilibrium constant in reactions which have a limiting reagent?

If in the following reaction

$$\ce{H2 + I2 <=> 2HI}$$

the initial concentrations of $$\ce{H2}$$ and $$\ce{I2}$$ are $$\pu{1 M}$$ and $$\pu{5 M}$$, respectively, and at equilibrium $$x$$ moles of each have combined. Then what will be the $$K_\mathrm{c}$$ for the reaction?

What I think is that the $$K_\mathrm{c}$$ will not change because it does not depend on the initial concentrations of reactants and depends only on the stoichiometric equation, but the one thing I can't reason out is that those extra $$4$$ moles of $$\ce{I2}$$ should also have an affect on its equilibrium concentration (since those $$4$$ moles are still present in the vessel) and thus affect $$K_\mathrm{c}$$.

• If $x$ moles have combined, then there must be $2x$ moles of $\ce{HI}$, $1-x$ moles of $\ce{H2}$ and $5-x$ moles of $\ce{I2}$, so therefore $K_c$ is ... – Curt F. Dec 18 '18 at 1:25
• @IvanNeretin By "change" I mean the deviation from the ideal condition where moles of $\ce{H2}$ and $\ce{I2}$ are given according to the balanced equation ( 1 each in this case). – Shashwat Saxena Dec 18 '18 at 5:54