4.5 moles each of hydrogen and iodine were heated in a sealed ten litre vessel. At equilibrium 3 moles of $HI$ were found. The equilibrium constant of $H_2\left(g\right)+I_2\left(g\right)\rightleftharpoons 2HI\left(g\right)$ is

                                $H_2+I_2\rightleftharpoons 2HI$

Initial moles      4.5       4.5       0

Moles at eqm   3          3          3

$K=\frac{{\left(3\right)}^2}{3\times 3}=1$ .

Note : when there is no change in the number of moles, $\Delta n=0$ the volume or capacity of vessel can be ignored.