Question

If molality of solution tends to zero what is the value of delta tf/m

Answer 1

The freezing point Tf of a solution of a nonvolatile solute is lessthan that of the pure solvent. The change in freezing point DeltaTf (also called the freezing point depression, Td) is given by:

Delta Tf = (Tf of pure solvent) - (Tf of solution)

Studies have shown that:

Delta Tf also equals Kf * (molality of solution)

where Kf is the freezing point depression constant (which dependsonly on the solvent, and not on the solute)

and molality of a solution is given by:

molality = (moles of solute)/(kg of solvent)

Thus the molecular weight of an unknown organic compound can bedetermined by measuring the freezing point depression of an aqueoussolution of the compound.

Using water as the solvent, the molecular weight of the unknownsolute can be calculated according to the following equations:

For water, Kf = 1.86 Deg C per molal, and the molal concentrationof the solute is given by:

molality = (moles of solute) / (kg solvent)

= ((grams solute) / MW) / (kg solvent)

where MW is the molecular weight of the solute.

To determine the molecular weight of an unknown from the freezingpoint depression, the freezing points of the pure solvent and ofthe solution are measured. Delta Tf is the difference between them,and the molality of the solution is then calculated from theequation for Delta Tf using the Kf constant.

The molecular weight of the solute, in g/mole, is obtained byrearranging the formula for molality to give the MW as:

MW = ((grams solute) / molality) / kg solvent

In this experiment you will determine the molecular weight of anunknown substance by measuring the freezing point depression of anaqueous solution of the unknown.

The liquid may cool to a temperature below the equilibrium freezingpoint. This is referred to as "supercooling". Supercooling can beminimized by continuously stirring the liquid as it cools. At thefreezing point the temperature remains constant because as theliquid solidifies, heat equal to the enthalpy of fusion isreleased.

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