The freezing point depression of a solution is directly proportional to the molality of the solute. The freezing point depression of a solution is given by the equation ΔTf = Kf * molality, where ΔTf is the change in freezing point, Kf is the freezing point depression constant of the solvent and molality is the concentration of the solute.
We know that the solid depressed the freezing point of stearic acid (Kf =4.5 °C/molal) by 3.5 °C, and that the same solid was dissolved in water (Kf = 1.86 C/molal) to the same final molality as it had in the stearic acid.
Thus, we can say that the final molality of the solid = 3.5 / 4.5 = 0.777 mol/kg
Now we need to find the freezing point depression for the same solution in water, so we use the freezing point depression constant of water (Kf = 1.86 C/molal) and the final molality of the solid in water
ΔTf = Kf * molality
ΔTf = 1.86 * 0.777 = 1.44 C
So the change in freezing point would be 1.44 C.