Question

In a freezing point-depression experiment where an unknown molecular compound is added to cyclohexane, a student collects the following data.

Mass of cyclohexane: 3.497772 g

Mass of unknown sample: 0.031104 g

Molality of unknown sample: 0.07157 mol/kg

Compute the molar mass in g/mol of the unknown sample from the studen'ts data. watch significant figures

Answer 1

we know that

molality = moles of unknown compound / mass of cyclohexane (kg)

so

moles of unknown compound = mass of cyclohexane (kg) x molality

given

molality = 0.07157 mol/kg

mass of cyclohexane = 3.497772 g = 3.497772 x 10-3 kg

so

moles of unknown compound = 3.497772 x 10-3 kg x 0.07157 mol/kg

moles of unknown compound = 2.5033 x 10-4 mol

now

molar mass = mass / moles

so

molar mass of unknown compound = 0.031104 g / (2.5033 x 10-4 mol)

molar mass of unknown compound = 124.25 g/mol