Question

Explain the effect of temperature on solubility of solid in liquid. Give the formula of van’t Hoff factor which relates the abnormal molecular mass to normal molecular mass. How does it affect by the association and dissociation?

Answer 1

Effect of temperature on solubility of solid in liquid. 

There is following equilibrium between soluble solid and solution in a saturated solution. Insoluble solid + Solute – Solvent Solution ⇌ Solution ;
∆H_sol = ± x k cal
According to Le-chatelier’s principle if ∆H > 0 (zero) i.e. on dissolving solute in solvent, energy is absorbed, hence on increasing temperature the solubility of solid solute increases .
Example: NH₄ CI, KCI, AgNO₃, NaNO₃, KI etc.
If ∆H < 0 (zero) i.e. on dissolving solute in solvent energy is released, hence on increasing temperature the solubility of solid solute decreases.
Example: NaOH, Li₂SO₄, (CH₃ COO)₃ Ca etc.
van’t Hoff’s factor

If observed colligative property < calculated value or M_o > M_c (or i<1), there is association. If observed colligative property > calculated value or Mo < Mc (ori >1), there is dissociation.