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Question

Define ebullioscopic constant?


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Solution

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 Kb.
  • 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, Tb=Tb-T°b

For example:

The boiling point of a solution of sodium chloride (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 (ΔTb​) for dilute solutions is directly proportional to the molal concentration of solute in the solution.

Therefore, we can write:

Tb=i×Kb×mwhere,i=Van'tHofffactorKb=ebullioscopicconstantm=molalityofthesolute


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