Temperature ∆ T b T f T f T b T b Freezing lines of solid solvent Freezing lines of solid solvent Figure . Elevation of boiling point and depression in freezing point The vapour pressure of the solution increases with increase in temperature as shown in the above figure. The variation of vapour pressure with respect to temperature for pure water is given by the violet coloured curve. At ⁰C the vapour pressure of water is equal to atm.
Hence the boiling point of water is ⁰C (T b °). When a solute is added to water, the vapour pressure of the resultant solution is lowered. The variation of vapour pressure with respect to temperature for the solution is given by green curve. From the graph, it is evident the vapour pressure of the solution is equal to atm pressure at the temperature T b which is greater than T b °.
The difference between these two temperatures (T b -T b °) gives the elevation of boiling point. The elevation of boiling point (ΔT b )= T b - T b ° The elevation of boiling point is directly proportional to the concentration of the solute particles. ΔT b α m ------ ( . ) m is the concentration of solution expressed in molality.
ΔT b = K b m ------ ( . ) Unit Unit - - - - Where K b = molal boiling point elevation constant or Ebullioscopic constant. If m= , then ΔT b =K b ; Hence, K b is equal to the elevation in boiling point for molal solution. K b is calculated by the following expression RT M b solvent vapourisation = ∆ Determination of molar mass of solute from elevation of boiling point If the solution is prepared by dissolving w B g of solute in w A g of solvent, then the molality is, m Number of moles of solute weight of solvent in grams × .............
............. Number of moles of solute w M ( . ) (