4.5 moles each of hydrogen and iodine are heated in a sealed 10 litre vessel. At equilibrium 3 moles of hydrogen iodide was found. The equilibrium constant for ${\mathrm{H}}_{2\left(\mathrm{g}\right)}+{\mathrm{I}}_{2\left(\mathrm{g}\right)}\rightleftharpoons 2{\mathrm{HI}}_{\left(\mathrm{g}\right)}$ is