Mercury is considered for possible regulation in the electric power industry under Title III of the 1990 Clean Air Act Amendments. One promising approach for removing mercury from fossil-fired flue gas involves the direct injection of activated carbon into the gas. Meserole et al. (1999) describe a theoretical model for estimating mercury removal by the sorbent injection process. An important parameter of the model is the effective diffusivity of mercuric chloride vapor traces in the flue gas. If the flue gas is at $1.013 \mathrm{bar}$ and $408 \mathrm{~K}$, and its composition (on a mercuric chloride-free basis) is $6 \% \mathrm{O}_{2}, 12 \% \mathrm{CO}_{2}, 7 \% \mathrm{H}_{2} \mathrm{O}$, and $75 \% \mathrm{~N}_{2}$, estimate the effective diffusivity of mercuric chloride in the flue gas. Assume that only the $\mathrm{HgCl}_{2}$ is adsorbed by the activated carbon. Meserole et al. reported an effective diffusivity value of $0.22 \mathrm{~cm}^{2} / \mathrm{s}$.