To find the degree of dissociation of gaseous HI at equilibrium, we need to know the initial and equilibrium concentrations of the reactants and products. However, you haven't provided the initial concentrations of HI, H2, and I2. Without that information, it is not possible to determine the degree of dissociation.
The degree of dissociation, symbolized by the Greek letter α, represents the fraction of a particular species that has dissociated or ionized. It can be calculated by comparing the equilibrium concentration of the dissociated species with the initial concentration of the undissociated species.
If you provide the initial concentrations or any additional relevant information, I can assist you further in calculating the degree of dissociation.