The Raoult's law reference article from the English Wikipedia on 24-Apr-2004
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Raoult's law

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In chemistry, Raoult's law states that the vapour pressure of mixed liquids is dependent on the vapour pressures of the individual liquids and the molar fraction of each present.

Once equilibrium has been reached,

PTotal = P1x1 + P2x2
where P1and P2 are the vapour pressures of two liquids, and x1 and x2 are their molar fractions.

This law is strictly valid only under the assumption that the bonding between the two liquids is equal to the bonding within the liquids. Therefore, comparing actual measured vapour pressures to predicted values from Raoult's law allows information about the relative strength of bonding between liquids to be obtained. If the measured value of vapour pressure is less than the predicted value, fewer molecules have left the solution than expected. This is put down to the strength of bonding between the liquids being greater than the bonding within the individual liquids, so less molecules have enough energy to leave the solution. Conversely, if the vapour pressure is greater than the predicted value more molecules have left the solution than expected, due to the bonding between the liquids being less strong than the bonding within each.

See also: Henry's law