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Raoult's law (/ ˈ r ɑː uː l z / law) is a relation of physical chemistry, with implications in thermodynamics.Proposed by French chemist François-Marie Raoult in 1887, [1] [2] it states that the partial pressure of each component of an ideal mixture of liquids is equal to the vapor pressure of the pure component (liquid or solid) multiplied by its mole fraction in the mixture.
where P A and P B are the partial vapour pressures of the two constituents and x A and x B are the mole fractions of the liquid. The equation gives the relation between changes in mole fraction and partial pressure of the components.
This is illustrated in the vapor pressure chart (see right) that shows graphs of the vapor pressures versus temperatures for a variety of liquids. [7] At the normal boiling point of a liquid, the vapor pressure is equal to the standard atmospheric pressure defined as 1 atmosphere, [1] 760 Torr, 101.325 kPa, or 14.69595 psi.
Mole fraction is numerically identical to the number fraction, which is defined as the number of particles of a constituent N i divided by the total number of all molecules N tot. Whereas mole fraction is a ratio of amounts to amounts (in units of moles per moles), molar concentration is a quotient of amount to volume (in units of moles per litre).
Binary mixture VLE data at a certain overall pressure, such as 1 atm, showing mole fraction vapor and liquid concentrations when boiling at various temperatures can be shown as a two-dimensional graph called a boiling-point diagram. The mole fraction of component 1 in the mixture can be represented by the symbol x 1.
Fractionation at total reflux. The Fenske equation in continuous fractional distillation is an equation used for calculating the minimum number of theoretical plates required for the separation of a binary feed stream by a fractionation column that is being operated at total reflux (i.e., which means that no overhead product distillate is being withdrawn from the column).
Here is the vapor pressure of component above the solution, is its mole fraction and is the vapor pressure of the pure substance at the same temperature. [ 5 ] [ 6 ] [ 7 ] This definition depends on vapor pressure, which is a directly measurable property, at least for volatile components.
These relationships are related to each other through the Gibbs–Duhem equation. [2] Note that in general activity coefficients are dimensionless. In detail: Raoult's law states that the partial pressure of component B is related to its vapor pressure (saturation pressure) and its mole fraction in the liquid phase,