# Vapour pressure

Vapour pressure of a liquid is the pressure exerted by the vapour in equilibrium with the liquid at a specific temperature.

The vapour pressure of a solvent is constant at constant temperature. But when a non-volatile solute is dissolved in it, the vapour pressure is lowered and the lowering of vapour pressure is proportional to the amount of solute dissolved.

Let ‘p’ be the vapour pressure of pure solvent and pS be that of solution, the lowering of vapour pressure will be $p- p_s$

The ratio of the lowering of vapour pressure $(p- p_s)$ to the vapour pressure of the solvent (p) is known as relative lowering of vapour pressure and is expressed as $(p- p_s) / p$

Relative lowering of V.P. = $\dfrac{P- P_s}{P}$

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