Define boiling point and find out expression for the molecular mass of non -volatile solute from the elevation of boiling point.

Explanation

Boiling point of a liquid is that temperature at which the pressure on its surface is equal to the atmospheric pressure. When a non - volatile solute is added tot he solvent the vapour pressure of the solution decreases. s such the temperature at which vapour pressure becomes equal to atmospheric pressure increases. In other words the boiling point of hte solution increases. The elevation in boiling point is represented as `Delta T_(b)` . Mathematically

`Delta T_(b) = T_(b) - T_(b)^(@)`
where `T_(b)` = Boling point of solution
`T_(b)^(@)` boiling point of solvent
This can also be shown graphically as follows :
It has been found that elevationin boiling point `(Delta T_(b))` is directly proportionalto the molaliry of the solutions ,
`Delta T_(b) = K_(b) m `
Here `K_(b)` is the molal elevation constant. It is defined as the elevation in boiling point when solution is one molar
`m = (" No of moles of solute")/( " Weight of solvent in kg") = (W_(B) xx 100)/( M_(B) xx W_(A))`
`Delta T_(b) = (K_(b) xx W_(b) xx 1000)/( M_(B) xx W_(A))`
or `M_(B) = (K_(b) xx W_(B) xx 1000)/( Delta T_(b) xx W_(A))`
From the above equation, molecular mass of solute can be calculated by knowing the values of other quantities