Chapter 15, Problem 11STP

(a)

Interpretation Introduction

Interpretation:

The reason of increase in boiling point on adding a solute to the solvent is to be explained.

Concept introduction:

Boiling point is the temperature when vapour pressure of liquid becomes equal to pressure of atmosphere.

Answer to Problem 11STP

Because adding the impurities increases the boiling point of solvent.

Explanation of Solution

When a solute is added to the solvent, some surface molecules of solvent are replaced by solute particles. So, the number of molecules which now vaporise will be less than pure solvent. So, to make the vapour pressure equal to atmospheric pressure, more temperature is to be provided. Hence, the boiling point of the solvent increases.

(b)

Interpretation Introduction

Interpretation:

The increase in boiling point on adding table salt compared to table sugar in a given amount of water is to be explained.

Concept introduction:

The rise in boiling point of a solvent on adding solute varies as:

  $\Delta T_b=k_b\times m$

Where

  $\Delta T_b$ is increase in boiling point

  $k_b$ is molal elevation constant

  $m$ is molality of solution.

Molality of solution is the number of moles of solution dissolved in given mass of solvent taken in kilograms.

Answer to Problem 11STP

Because table salt undergoes dissociation and table sugar doesn’t undergo dissociation.

Explanation of Solution

The number of moles of table salt and table sugar are same and volume of solvent taken is also same but rise in boiling point of table salt is more than table sugar because table salt undergoes dissociation in water as:

  $NaCl\to N{a}^{+}+C{l}^{-}$

Therefore, 1 mol $NaCl$ produces total 2mol ions.

But table sugar is a non-electrolyte.

As the rise in boiling point is directly proportional to number of moles, the rise is more for table salt as compared to table sugar.