"An azeotrope is a mixture of two or more liquids (chemicals) in such a ratio that its composition cannot be changed by simple distillation. Here is the begining of the Wikipedia entry: You might be interested to know that there are some solutions that resist distillation. That discussion is for another time, another class. However, please be aware that the reality is much more complex than the discussion just above. It is an extremely important research and industrial process. The general name of the separation process that exploits volatility differences is distillation. This fact allows us the means to separate two volatile components in a solution and obtain each substance in a (relatively) pure form. However, what happens then is that we would remove (from the solution) more and more of the component with the higher vapor pressure. If we were to constantly sweep away the vapor from above the solution, then more of the solution would vaporize. The component with the higher vapor pressure? There's more of it in the vapor (expressed in mole fractions) than in the solution. In our example, the acetone in the solution had χ = 0.64, but acetone's mole fraction in the vapor equals 0.81. (By the way, we know that pressure is directly proportional to the number of moles from consideration of the Ideal Gas Law.)Ī key point about the above result is this: the vapor is richer in the component with the higher vapor pressure than the solution. This is done by dividing each component's vapor pressure by the total vapor pressure. This is important because it allows us to calculate the composition (expressed using mole fractions) of the vapor. It is that we also know the vapor pressures of the two components of the vapor. There is something very important about the above calculation for the vapor pressure of the solution. P solution = ( P cy o) ( χ cy ) + ( P ac o) ( χ ac )
What is P solution, the total vapor pressure, above this solution?ġ) Calculate the mole fraction of each substance:Ĭyclohexane ⇒ 1.40 mol / (1.40 mol + 2.50 mol) = 0.358974359Īcetone ⇒ 2.50 mol / (1.40 mol + 2.50 mol) = 0.641025641 The subscripts A and B stand for the two different volatile substances in the solution.Įxample #1: A solution is composed of 1.40 mol cyclohexane ( P cy o = 97.6 torr) and 2.50 mol acetone ( P ac o P solution = ( P A o) ( χ A ) + ( P A o) ( χ B ) The key equation to use is Raoult's Law, but in a slightly expanded form from how it is first presented: We will discuss the real behavior of solutions in a different tutorial. Also, note the presence of the word 'ideal' in the title. More complex solutions (with three or more volatile components) are discussed at a level beyond the scope of the ChemTeam's mission. 100% of the nonvolatile solute stays in solution, none of it enters the vapor above the solution.īy the way, at this introductory level, we will only discuss solutions with two volatile components.
In a solution with a nonvolatile solute, only the pure vapor of the solvent is present above the solution. The key point to remember about solutions with two (or more) volatile components? All the components are represented in the vapor that is in contact with the solution. Raoult's Law: Vapor Pressure and Volatile Solutes (in Ideal Solutions) Raoult's Law: Vapor Pressure and Volatile Solutes (in Ideal Solutions)
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