The addition of $115\mathrm{.}0$ g potassium fluoride to $609\mathrm{.}0$ mL of water $($d $=1\mathrm{.}00$ g$/$mL$)$ causes the temperature to rise $3\mathrm{.}81\backslash$deg C$.$ What is the $\backslash$Delta Hrxn for KF$?($Assume cs $=4\mathrm{.}18$ J$/$g oC$.)$