what is the freezing point of a 6.5 molal glucose solution

Question

finding the freezing point of this glucose solution

Answer 1

Glucose is not ionic and does not dissociate, thus a 6.5 molal solution of glucose simply has 6.5 moles of solute per kg of water, by the definition of molality. Adding solute to water decreases the freezing point and is an example of a colligative property. Usually we estimate that the change in freezing point for the water only depends on the concentration of solute particles.

 

We use Blagden's Law to determine the change in freezing point (ΔT_F):

ΔT_F = K_F · b · i

 

The Van't Hoff factor (i) will be 1, because glucose does not break apart or associate. The molality (b) is 6.5 molal. We look up the cryoscopic constant (K_F) for water: 1.853 K·kg/mol.

 

Thus, ΔT_F = (1.853) · (6.5) · (1) = 12.044 K.

 

We remember that the freezing point goes down, and that pure water normally freezes at 0 °C, and that a step of Kelvin is equivalent to a step of Celcius, and that we should round to two sig figs, so:

 

The 6.5 molal solution of glucose freezes at -12 °C.