Chemistry - 74 - van’t Hoff factors?

Calculate the freezing point and the boiling point of each of the following solution using the observed van’t Hoff factors.
a.0.050m MgCl2
b.0.050m FeCl3

Answer:
The boiling point elevation is characteristic for each solvent ; it is proportional to the molal concentration of solute particles diisolved therein.
For H2O kb = 0.512 °C.
In the same way that dissolved particles rause the b.p. proportioanal to their molal concentration they depress the freezing point as well.
For H2O kf = 1.86 °C

MgCl2 >> Mg2+ + 2Cl- ( we get 3 ions so 3 is the Van'Hoff factor)
delta T = 0.050 x 3 x 1.86 = 0.279 . The f.p. of the solution is
-0.279 °C
deltaT = 0.050 x 3 x 0.512 = 0.0768 °C. The b.p. of the solution is 100.0768 °C

FeCl3 >> Fe3+ + 3Cl- ( 4 ions)
deltaT = 0.050 x 4 x 1.86 = 0.372 °C . f.p. = - 0.372 °C
deltaT = 0.050 x 4 x 0.512 = 0.102 °C . b.p. = 100.102 °C

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