The freezing point of pure benzene was measured as 5.49 °C.With the same benzene and the same equipment,the freezing point of a solution containing 6.60 g of an unknown molecular compound in 140 g of benzene was found to be 2.35 °C.The cryoscopic constant,K_f,for benzene is 5.12 °C kg/mol.Calculate the mole weight of the unknown. A)103 g/mol B)76.9 g/mol C)44.0 g/mol D)28.9 g/mol E)17.4 g/mol

Question

Quizplus · Accepted Answer

First, we need to calculate the molality (m) of the solution:

ΔTf = Kf × m
m = ΔTf / Kf
m = (5.49 °C - 2.35 °C) / (5.12 °C kg/mol)
m = 0.61 mol/kg

Next, we need to calculate the moles of solute (n):

n = m × kg solvent
n = 0.61 mol/kg × 0.140 kg
n = 0.0854 mol

Finally, we can calculate the molecular weight (MW) of the unknown compound:

MW = molar mass / number of moles
MW = (6.60 g / 0.0854 mol)
MW = 77.3 g/mol

Therefore, the best choice is B) 76.9 g/mol, which is the closest answer to our calculated result.

The Freezing Point of Pure Benzene Was Measured as 5