Set up a reaction table and use the variables to find equilibrium concentrations in the equilibrium expression.
Set up equilibrium expression:
K_\text{c}=1.26\times 10^{-3}=\frac{\text{[H}_2]\text{[I}_2]}{\text{[HI}]^2} =\frac{\text{[x][x]}}{[0.242248-2\text{x}]^2}\quad\quad \text{Take the square root of each side} \\ 0.035496=\frac{\text{[x]}}{[0.242248-2\text{x}]} \\ 0.0085988-0.070992\text{x = x} \\ \text{x}=8.0288\times 10^{-3}=8.03\times 10^{-3} \\ \text{[H}_2]=\text{[I}_2]=8.03\times 10^{-3} \ MCheck: Plug equilibrium concentrations into the equilibrium expression and calculate K_\text{c}.
\text{[HI]} = 0.242 – 2(8.03 \times 10^{-3}) = 0.226 \ M \\ (8.03 \times 10^{-3})^2/ (0.226)^2= 1.2624 \times 10^{-3}= 1 .26 \times 10^{-3}