In my textbook, it is written that lowering of vapour pressure is not a colligative property but relative lowering of vapour pressure is a colligative property. But why is that? There is literally no explanation given in my textbook as to why this is true. Please someone explain. I am so confused. Please help. (By lowering of vapour pressure I mean $P_{0} - P_{S}$ where $P_{0}$ is the vapour pressure of pure solvent and $P_{S}$ is the vapour pressure of the solvent containing non-volative solute. Relative Lowering in Vapour Pressure means $\frac{P_{0} - P_{S}}{P_{0}}$).

  • $\begingroup$ Could you clarify with respect to the difference between 'relative lowering' and 'lowering'? $\endgroup$
    – Jon Custer
    May 6 at 18:27
  • 3
    $\begingroup$ Consider reporting the whole sentences $\endgroup$
    – Alchimista
    May 7 at 9:10
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    $\begingroup$ Yeah, like @Alchimista said, please give us the complete quote; plus please include a full citation to the textbook (author, name, publication year, publisher, edition, etc.). $\endgroup$
    – theorist
    Jun 23 at 4:23

A colligative property is a property which depends on number of solute particles and not on nature of solute particles. Relative lowering in vapour pressure is equal to the mole fraction of the solute . This can be derived using $P_s=P_0\cdot x_\text{solvent}$ where $x$ is the mole fraction and Po is the vapour pressure of pure solvent. Lowering of vapour pressure is also a colligative property as it is equal to $\frac{x_\text{solute}}{P_0}$. $P_0$ is a property of the solvent, not the solute.


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