What mass of naphthalene must be dissolved in 300.g of benzene to produce a solution with a freezing point ?

Question

2.5 degrees below that of pure benzene? Kf for benzene is 5.12 degreeC kg/mol.

Answer 1

dTemp = Kf (molality)

2.5 = 5.12 (molality)

molality = 0.488 molal

which is 0.488 moles  naphthalene per 1 kg solvent

which is (0.488 mol) (128.2 g/mol Naph) = 62.6 grams of Naph per kg solvent

? g naph = 0.300kg  benz @ 62.6 grams naph / 1 kg benz =

your answer:18.8 grams of naphthalene