You have an aqueous solution of NaCl that has a freezing point of $-5\mathrm{.}25C.$ What mass $($in grams$)$ of NaCl must be added to $1000g$ of this solution to lower the freezing point to $-12\mathrm{.}00C?($Assume a van't Hoff factor of $1\mathrm{.}9$ for NaCl $).(Kf$ for water is $1\mathrm{.}86C*k\frac{g}{?}$ mol$).$