Elevation In b.pt. Of The Solvent (Ebullioscopy)
Category : JEE Main & Advanced
Boiling point of a liquid may be defined as the temperature at which its vapour pressure becomes equal to atmospheric pressure, i.e., 760 mm. Since the addition of a non-volatile solute lowers the vapour pressure of the solvent, solution always has lower vapour pressure than the solvent and hence it must be heated to a higher temperature to make its vapour pressure equal to atmospheric pressure with the result the solution boils at a higher temperature than the pure solvent. Thus sea water boils at a higher temperature than distilled water. If Tb is the boiling point of the solvent and T is the boiling point of the solution, the difference in the boiling point (DT or D Tb) is called the elevation of boiling point.
\[T-{{T}_{b}}=\Delta {{T}_{b}}\] or \[\Delta T\]
Elevation in boiling point is determined by Landsberger’s method and Cottrell’s method. Study of elevation in boiling point of a liquid in which a non-volatile solute is dissolved is called as ebullioscopy.
(1) The elevation of boiling point is directly proportional to the lowering of vapour pressure, i.e., \[\Delta {{T}_{b}}\propto {{p}^{0}}-p\]
(2) \[\Delta {{T}_{b}}={{K}_{b}}\times m\]
where \[{{K}_{b}}=\] molal elevation constant or ebullioscopic constant of the solvent; \[m=\] Molality of the solution, i.e., number of moles of solute per \[1000g\] of the solvent; \[\Delta {{T}_{b}}=\] Elevation in boiling point
(3) \[\Delta {{T}_{b}}=\frac{1000\times {{K}_{b}}\times w}{m\times W}\] or \[m=\frac{1000\times {{K}_{b}}\times w}{\Delta {{T}_{b}}\times W}\]
where, \[{{K}_{b}}\] is molal elevation constant and defined as the elevation in b.pt. produced when 1 mole of the solute is dissolved in 1 kg of the solvent.
\[w\] and \[W\] are the weights of solute and solvent and \[m\] is the molecular weight of the solute.
(4) \[{{K}_{b}}=\frac{0.002{{({{T}_{0}})}^{2}}}{{{l}_{V}}}\]
where \[{{T}_{0}}=\] Normal boiling point of the pure solvent; \[{{l}_{V}}=\]Latent heat of evaporation in \[cal/g\] of pure solvent; \[{{K}_{b}}\] for water is \[0.52\ \deg -kg\ mo{{l}^{-1}}\].
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