FREEZING POINT DEPRESSION
The freezing point of a solution is lower than the freezing point of the pure solvent.
This phenomenon is referred to as freezing point depression.

The equation describing freezing point depression is: DT = (K_f)(m)

    DT represents the freezing point depression: DT = T^o_f - T_f
               T^o_f is the freezing point of the pure solvent.
                DT_f is the freezing point of the solution.

    K_f  is the molal freezing point depression constant for the solvent.
                The value of K_f is characteristic for a given solvent.
                The units of K_f are given in ^oC/m.

    m is the molality of the solution.
            m = the number of moles of solute per kg of solvent.
            m = moles of solute/ kg of solvent
 

EXAMPLE 1
What is the molality of a solution containing 27.35 grams of NaCl in 575 grams of H₂O ?
[0.814 m]
 

EXAMPLE 2
How many grams of sucrose (C₁₂H₂₂O₁₁) are needed to mix with 500 g of water to give a 0.100 m solution?
[17.1 g sucrose]
 

EXAMPLE 3
What is the freezing point (T_f ) of a 0.500 m aqueous solution of glucose?
(K_f for H₂O is 1.86^oC/m)
[T_f = - 0.93 ^oC]
 

EXAMPLE 4
A solution containing 7.24 g of a nonvolatile compound dissolved in 115.3 g of benzene froze at 3.55^oC.
What is the molecular weight of the solute?
Pure benzene freezes at 5.45^oC and K_f for benzene is 5.08 ^oC/m.
[MW = 168 g/mol]