Home ice cream makers use ice to cool a concentrated NaCl solution below the normal freezing point of water. What molal concentration of salt is needed to lower the freezing point to -4.25 C? Assume that the solution is ideal, and that the van't Hoff factor for NaCl is the expected (ideal) value.
Kf for water = 1.86°C m.kg
If you have a solution that is 1m in particles , then the aqueous solution will freeze at -1.86° C.
Your solution freezes at -4.77°C
Therefore the solution is 4.77/1.86 = 0.9516 molal in particles
Because NaCl dissociates to form 2 ions, ie van't Hoff factor i = 2, the solution must be 0.9516/2 = 0.4708m in respect of NaCl. That is the answer.