A $956\mathrm{.}8mL$ aqueous solution of $502\mathrm{.}5mMN{a}_{2}S$ is combined with $100\mathrm{.}0mL$ of an aqueous $HCl$ solution whose $HCl$ concentration is not known. The reaction produces $2\mathrm{.}521L$ of $H_{2}S$ gas at a pressure of $1\mathrm{.}00$atm and a temperature of $6\mathrm{.}78C.$ There is some $N{a}_{2}S$ remaining after the reaction is complete. What is the concentration of sulfide ions in the solution after the reaction has completed? Assume the volumes of the solutions are additive. Express your answer in units of millimolar $($mM$)$ using at least three significant digits.