At a certain temp. 2HI H2 + I2 Only 50% HI is dissociated at equilibrium. The equilibrium constant is

2 2 2HI H I 2 0 0 2 50 50 50 2 100 100 100 1 0.5 0.5 x CHEMICAL EQUILIBRIUM 40 2 2 C 2 H I 0.5×0.5 K = = = 0.25 1 HI