Question

How is Raults law different from Dalton's law of partial pressure

Answer 1

Dear student,“Dalton’s law states that in a mixture of non-reacting ideal gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.” Therefore, in a gas, If gas A has vapour pressure 5, and Gas B has vapour pressure 10, mixing both the gases will give you total pressure of 15. Simple. This works only for Ideal gases. PT=PA+PB The assumptions are that of ideal gas, no interaction between molecules, and gas atoms being infinitsimally small. “Raoult’s law states that the partial vapor pressure of each component of an ideal mixture of liquids is equal to the vapor pressure of the pure component multiplied by its mole fraction in the mixture.” Pure Liquid A has the vapour pressure 5. Pure liquid B has vapour pressure 10. When both are mixed, unlike gases their vapour pressure do not add up. It may be because can gases mix up freely while liquid have to evaporate from a surface. If there is 50% liquid A and 50% liquid B, we can assume that the surface has 50 % liquid A molecules and 50% liquid B molecules lined up and thus only 50% molecules can evaporate at a time. Thus vapour pressure is only 50% of the vapor pressure that the liquid will have if provided the entire surface (Pure Liquid). We can mix different % of liquid A and liquid B to form a solution. Thus it is dependent on % ie. the concentration (mole fraction). Thus Total vapour pressure is not PT=PA+PB but PT=xAPA+xBPB. Mole fraction is always less than 1 and is a fraction, thus we can imagine both fractional vapour pressure (of individual pure solvent) adding up. Regards.