Question

What is used in the calculation of boiling point elevation?
(A) Molarity.
(B) Molality.
(C) Mole Fraction.
(D) Density.
(E) Partial Pressure.

Answer 1

We know that boiling point elevation is a phenomenon which occurs when a non-volatile solute, which means a solute which does not evaporate easily, is added to a solvent. Degree of dissociation of a solute is one in case of solutes which don't dissociate when they are added to water, and more than one in case of ionic solutes, the exact value depends on which solute we are considering.

Complete answer:
Boiling point elevation can be defined as the increase in the boiling point of a solvent under observation, when solute is added to it. When a non-volatile solute, that means a solute which does not evaporate easily, is added to a preferred solvent, the solution as a result has a higher boiling point as compared to that of the pure solvent. For instance, the boiling point of a solution which consists of sodium chloride and water is greater than the boiling point of the pure water. Elevation of boiling point is one of the four colligative properties of matter, meaning, it is a property which is dependent on the ratio of solute-to-solvent but not on the identity of the solute. In other words we can say that the elevation in the boiling point of a solution is dependent on the amount of solute which is being added to it, and not the nature of that solute. The greater the concentration of solute in the solution, the higher the boiling point elevation will be. Molality is used in the calculation of boiling point elevation.
$\Delta {{T}_{b}}={{k}_{b}}\times m\times i$
$\Delta {{T}_{b}}=$ Elevation in boiling point,
${{k}_{b}}=$ Boiling point elevation constant,
$m=$ Molality and
$i=$ Van’t Hoff Factor
Therefore, the correct answer is option B.

Note:
Remember that since calcium chloride is an ionic molecule, so when it is dissolved in water as a solute, it dissociates into its constituent ions, which are calcium and chloride. Since, two chloride ions and one calcium ion are produced by one mole of calcium chloride, the value of the degree of dissociation becomes three.