Question

After you preform your experiment, you determine that the Kf value for naphthalene is 6.91 °C/m . You are using 10g of naphthalene and added 1.0 g of your unknown. The the freezing point of the solvent decreased by 4.49 °C°C when the unknown was added. Knowing this information, determine the molar mass of the unknown.

Answer 1

153.9 g/mol

Step-by-step explanation:

The freezing point depression for an ideal solution is given by the expresion:

ΔTf = Kf m

where:

ΔTf is the freezing point depression

Kf is the freezing constant of the solvent

m is the molalility which mol of solute per kilogram solvent

The moles of solute is the mass divided into its molecular weight, MW

substituting into the equation, we have:

4.49 º C = 6.91 ºC/m x [(1.0 g/MW) / 0.010 kg]

4.49 º C = 6.91 ºC/mol/kg x [(1.0 g/MW) / 0.010 kg]

⇒4.49 = 691 g/mol/MW

⇒ MW = 691 / 4.49 g/mol = 153.9 g /mol