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

$$\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g)$$

Initial: 4.5 mol, 4.5 mol, 0 mol in 10 L vessel.

At equilibrium: 3 mol HI formed → 1.5 mol each of H₂ and I₂ consumed.

$$[\text{H}_2] = \frac{3}{10}$$, $$[\text{I}_2] = \frac{3}{10}$$, $$[\text{HI}] = \frac{3}{10}$$

$$K_c = \frac{[\text{HI}]^2}{[\text{H}_2][\text{I}_2]} = \frac{(0.3)^2}{(0.3)(0.3)} = 1$$