4 . 5 moles each of hydrogen and iodine is heated in a sealed ten...

The equilibrium moles of each reactant and product can be calculated as follows,

${H}_{2}\left(g\right) + {I}_{2}\left(g\right) ⇌ 2{HI}_{\left(g\right)}initial4 . 54 . 50Atequilibrium4 . 5 - 1 . 54 . 5 - 1 . 53$

Now, using equilibrium moles, equilibrium constant calculated as shown below.
${K}_{eq} = \dfrac{[ HI{]}^{2}}{\left[{H}_{2}\right]\left[{I}_{2}\right]} = \dfrac{( 3{)}^{2}}{3 \times 3} = 1$