Colligative properties as elevation of boiling point part-4

Elevation of boiling point (Ebullioscopy) 

•  Boiling point – is the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure. 
•      Generally, we found that the B. P. of Solution is always higher than that of the pure solvent. the decrease in the boiling point of solution is called elevation of boiling point. 
• we know that vapour pressure of solvent (pure) diseases with mixing non-a volatile which occupy the surface of solvent, therefore, vapour pressure of solution is lower than that of the pure Solvent. 
• The variation of vapour pressure of pure solvent and solution is as a function of temperature. 
• hence the solution has to be heated more to 
• make the vapour pressure equal to the atmospheric 
• Elevation of boiling point may be explained on the basis of graph plotted between vapour pressure & Temperature. 

let us consider consider T°b as boiling point of pure solvent & Tb   as boiling poiling poin of 
solution.  Thus, the increase in boiling point is      
ΔTb  = Tb -   T°b          …..............(1)               

Excrementally, elevation of boing point for dilute solution is directionally proportional to molal concentration to solution   

ΔTb  α   m   …...............(2)     

 (here m= molality = w2 x 1000 /M2 x w1   , w2 = mass of solute    M2 = molar mass of solute 
w1   = mass of solvent 
ΔTb  =kbm   ------------(3)
ΔTb  =kb w2 x 1000 /M2 x w1 …. (4)                        

So   M2 =kb w2 x 1000 / ΔTb  x w1   …..(5)      

this equation is used to calculate molar mass of solute dissolved in solution. 

Kb is boiling point elevation constant or molal elevation constant or Ebullioscopic constant   which depends upon nature of solvent. The relationship between  Kb and vaporisation of enthalpy may be given as -           

kb  = R xM1 x  (T°b )2   /   1000 x  ΔvapH    …...... (6)

Problem - 1