The pH of which salt is independent of its concentration ?

If $0.5mollitr{e}^{-1}$ of $H{I}_{(g)}$ is present at equilibrium at $700K$, what are the concentrations of $H_{2(g)}$ and $I_{2(g)}$ assuming that we initially started with $H{I}_{(g)}$ and allowed it to reach equilibrium at $700K$.