Question

Elevation of boiling point is a colligative property what are colligative properties and elevation of boiling point is directly proportional to molality of solution does Delta T V is equal to KB into a KB is called the molal elevation constant from the above relation derive an expression to obtain molar mass of the solute

Answer 1

The properties of a solutions which depend upon no. of solute particles present in a definite amount of solvent, but not on the chemical nature of the solute are called colligative properties.

According to Raoult's Law, for a dilute solution,

$\frac{p° -p}{p° }$ =$\frac{wM}{WM'}$

= Δ$\frac{p}{p°}$ = $\frac{wM}{WM'}$

Since, p°M is constant for a particular solvent,

therefore, Δ$T_{b}$  is directly proportional to $\frac{w}{WM'}$

if W = 1kg,  $\frac{w}{WM'}$ is molality of the solution.

so,

Δ$T_{b}$ is directly proportional to m

therefore,

Δ$T_{b}$ = Δ$K_{b}$  . m

the formula derived also written as

$K_{b}$ = $\frac{MRT_{b}^{2}  }{1000 * Δ H_{v} }$

where

Δ$H_{v}$ is molar enthalpy of vapourisation of solvent.