Question

Define ebullioscopic constant?

Answer 1

Ebullioscopic constant:

• The ebullioscopic constant is also called the molal elevation constant.
• It is defined as the elevation in boiling point when one mole of non-volatile solute is added to one kilogram of solvent.
• It is denoted by K_b.
• The unit of an ebullioscopic constant is K Kg mol^-1.

Elevation in boiling point:

• Boiling point elevation refers to the increase in the boiling point of a solvent upon the addition of a solute. When a non-volatile solute is added to a solvent, the resulting solution has a higher boiling point than that of the pure solvent.

Therefore, $∆{\mathrm{T}}_{\mathrm{b}}={\mathrm{T}}_{\mathrm{b}}-{{\mathrm{T}}^{°}}_{\mathrm{b}}$

For example:

The boiling point of a solution of sodium chloride ($\mathrm{NaCl}$) and water is greater than that of pure water.

• Boiling point elevation is a colligative property of matter, that is it is dependent on the solute-to-solvent ratio but not on the solute’s identity. This implies that the elevation in the boiling point of a solution depends on the amount of solute added to it. The greater the concentration of solute in the solution, the greater the boiling point elevation.
• Experimentally, it has been shown that the elevation of the boiling point (ΔT_b​) for dilute solutions is directly proportional to the molal concentration of solute in the solution.

Therefore, we can write:

$$\begin{gathered} ∆{\mathrm{T}}_{\mathrm{b}}=\mathrm{i}\times {\mathrm{K}}_{\mathrm{b}}\times \mathrm{m} \\ \mathrm{where},\mathrm{i}=\mathrm{Van}\text{'}\mathrm{t}\mathrm{Hoff}\mathrm{factor} \\ {\mathrm{K}}_{\mathrm{b}}=\mathrm{ebullioscopic}\mathrm{constant} \\ \mathrm{m}=\mathrm{molality}\mathrm{of}\mathrm{the}\mathrm{solute} \end{gathered}$$