Question

If 0.725 g of a hydrocarbon is dissolved in 45.0 g of cyclohexane the solution Freezers at a temperature of 4.0°C less than the pure solvent. What is a major mass of the unknown hydrocarbon? Kf = 20.0°C/m

Answer 1

Given : Decrease in freezing temperature = 4.0 ^oC

Molality of hydrocarbon = (Decrease in freezing temperature) / (K_f)

Molality of hydrocarbon = (4.0 ^oC) / (20.0 ^oC/m)

Molality of hydrocarbon = 0.20 m

mass of cyclohexane = 45.0 g = 0.0450 kg

moles of hydrocarbon = (molality of hydrocarbon) * (mass of cyclohexane in kg)

moles of hydrocarbon = (0.20 m) * (0.0450 kg)

moles of hydrocarbon = 9.0 x 10^-3 mol

molar mass hydrocarbon = (mass of hydrocarbon) / (moles of hydrocarbon)

molar mass hydrocarbon = (0.725 g) / (9.0 x 10^-3 mol)

molar mass hydrocarbon = 80.6 g/mol