The melting point of a pure solvent (kf=1.79°C/m) is -116.3°C. If 312 g of an ionic solid (i=2.6) is added to 567.7 g of the solvent, the new melting point is -101.6°C. What is the molar mass of the ionic solute?
delta T = i*Kf*molality
Substitute and solve for molality.
molality = moles/kg solvent. You have molality and kg solvent, solve for moles.
moles = grams/molar mass. You have moles and grams, solve for molar mass.