Raoult's law

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The Raoult law of François Marie Raoult is a linear approximation for the vapor pressure of liquid mixtures . According to this, the partial pressure in the gas phase for each component of the mixture is given by the product of the mole fraction of the component and the vapor pressure that the substance would have in its pure form.

Vapor pressures p A and p B (green) and total pressure (black) over different mixtures of two chemically similar substances A and B. In the horizontal direction, the molar proportions are plotted: left is x A = 1 and x B = 0, right is the other way around. The dashed courses correspond to Raoult's law.

In the case shown on the right, the total pressure is only slightly higher than calculated according to Raoult's law. Most real mixtures show a behavior that deviates more strongly upwards or downwards.

If all components except one (the solvent ) have a negligible vapor pressure, the vapor pressure drops proportionally to the total amount of substance of all dissolved substances, regardless of their type. The vapor pressure of the solvent is called a colligative property .

Formula representation

with partial pressures , vapor pressures of the pure substances and substance quantities for

See also

  • Henry's law (linear approximations at the ends of the curve, for a little )
  • Duhem-Margules equation (allows the curve to be curved for a better approximation in the transition area)

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