To determine the equilibrium constant Kc for the reaction, we first need to write the balanced chemical equation and the expression for Kc:H2 g + I2 g 2HI g Kc = [HI]^2 / [H2] * [I2] Next, we need to find the change in concentration for each species at equilibrium. Since the initial concentration of HI is not given, we can assume it to be 0 M. The equilibrium concentration of HI is given as 0.40 M, so the change in concentration for HI is +0.40 M.For the balanced equation, 1 mole of H2 reacts with 1 mole of I2 to produce 2 moles of HI. So, the change in concentration for H2 and I2 will be half of the change in concentration for HI:Change in [H2] = Change in [I2] = -0.40 M / 2 = -0.20 MNow, we can find the equilibrium concentrations for H2 and I2:[H2]_eq = 0.50 M - 0.20 M = 0.30 M[I2]_eq = 0.25 M - 0.20 M = 0.05 MFinally, we can plug these equilibrium concentrations into the Kc expression:Kc = [HI]^2 / [H2] * [I2] Kc = 0.40 M ^2 / 0.30 M * 0.05 M Kc = 0.16 M^2 / 0.015 M^2Kc 10.67The equilibrium constant Kc for the reaction of hydrogen gas and iodine gas to form hydrogen iodide gas at room temperature is approximately 10.67.