Raoult's law, in its most general form, states that the vapor pressure above an ideal solution is given by the sum of the products of the vapor pressure of each component and its mole fraction (the number of moles of the component divided by the total number of moles of all the components). Raoult's law holds only approximately and over a limited range of conditions, differing for different substances. It is used to find the relative molecular mass of a substance and has been fundamental to many theories of solution. It is named after the French physical chemist François Raoult (1830–1901). See also distillation.