Themes > Science > Chemistry > Miscellenous > Help file Index > Liquid and Solid Properties > Raoult's Law When two liquids are mixed, the vapor pressure of the resulting solution roughly follows Raoult's Law, which states that the vapor pressure of the mixture is a function of the vapor pressures of the individual components and their mole fractions: Ptot = PAXA + PBXBwhere PA and PB are the vapor pressures of the A and B components and XB and XB are the mole fractions. In reality, only mixtures of two similar liquids follow this law very closely: these are known as ideal solutions. Many solutions show either a higher or lower vapor pressure than expected. If the vapor pressure of a solution is lower than expected, then the intermolecular bonds between the A and B molecules are stronger than either the A-A or B-B bonds. Since the bonds are stronger, it takes more energy to break them and thus fewer molecules enter the gas phase than in an ideal solution, resulting in a lower vapor pressure. The reverse is also true: if the vapor pressure is higher than expected, then the A-B bonds are weaker than the A-A or B-B bonds. Thus, comparing experimental results to Raoult's law can tell us a lot about the bonding between molecules. Example: A mixture of 30.0 grams of ethyl acetate, CH3CH2COOH and 20.0 grams of water is heated to 27oC. At this temperature, the vapor pressure of ethyl acetate is 100 mmHg and the vapor pressure of water is 26.2 mmHg. What is the vapor pressure of the mixture? Solution: Use Raoult's Law to determine the vapor pressure of the mixture. For this, we need the mole fractions of each component, so start by computing the number of moles of each: 30.0 g ethyl acetate/74.08 g/mole = 0.404 moles ethyl acetate. 20.0 g water/18.02 g/mole = 1.11 moles waterThus, we have a total of 0.404+1.11 = 1.51 moles of stuff in the mixture. The mole fractions are then Xethyl acetate = 0.404/1.51 = .265 Xwater = 1.11/1.51 = .735 Now just use the equation and we're done Ptot = PEAXEA + PwaterXwater Ptot = 100 mmHg*.265 + 26.2 mmHg*.735 Ptot = 46.0 mmHg Information provided by: http://learn.chem.vt.edu