Question

Give reasons for the following.
Elevation of boiling point of 1M KCl solution is nearly double than that of 1M sugar solution.

Answer 1

Hint:When a solute is added to a solvent, the boiling point of the solution increases. It will be greater than the boiling point of pure solvent. Elevation of boiling point is a colligative property of solution and it depends on the number of solute particles present in the solution.

Complete step by step answer:
Elevation of boiling point depends on the number of solute particles present in the solution. In this case both KCl and sugar solution have some molarity. The van't Hoff factor for KCl is twice that of sugar. This is because sugar does not dissociate in water and is a non electrolyte. So, its van't hoff factor is 1. KCl can dissociate in water and is an electrolyte. So, 1 molecule of KCL gives 2 ions. Hence, its van't Hoff factor is 2.
  So, the elevation of boiling point of 1M KCl solution will be nearly double than that of 1M sugar solution.
Elevation in boiling point is given by the equation
\[\Delta {{T}_{b}}=i{{K}_{b}}m\]
Where \[\Delta {{T}_{b}}\]is the elevation in boiling point.
\[i\]is the van't hoff factor and \[{{K}_{b}}\] is the molal elevation constant and m is the molality.
Thus, from above expression we get \[\Delta {{T}_{b}}\propto i\]
Therefore, elevation of boiling point of 1M KCl solution is nearly double than that of 1M sugar solution.

Additional Information:
Elevation of boiling point is dependent on the presence of dissolved particles and their number, but not their identity. It can be explained in the terms of vapour pressure or chemical potential of the solvent. Elevation of boiling point is used to measure degree of dissociation or the molar masses of the solute.

Note:
Van’t Hoff factor is the ratio of the concentration of particles produced when the substance is dissolved and the concentration of the substance calculated using its mass. The van’t Hoff factor is always less than the value predicted using the ideal equation.