Chem 104 Freezing Point Depression

(See Moore , 2^nd ed., Sections pp. 743-744)

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 mathematical equation describing freezing point depression is:  ΔT = (K_f)(m)

ΔT represents the freezing point depression:  ΔT = T_f^o- T_f

T_f^o is the freezing point of the pure solvent.

T_f is the freezing point of the solution.

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

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

Example 1

What is the molality of a solution containing 27.35 grams of NaCl dissolved 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 of a 0.500 m aqueous solution of glucose?     (K_f for H₂O is 1.86^oC/m)

(- 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.