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Chem 112 Exam AID Course Pack

Vapour pressure of the liquid has dropped to be equal

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Unformatted text preview: case of temperature). Consider the water- acetone fraction graph with vapour pressure as the y- axis. - if the solution were ideal, the vapour pressure of water would rise linearly moving from 0 to 1, and the vapour pressure of acetone would fall linearly moving from 0 to 1 - the total pressure (which is the sum of the other two, by Dalton’s law) would move linearly as well Ethan Newton & Barry Zhang for SOS Winter 2012 23 - this is based on Raoult’s Law (the vapour pressure of each liquid is directly proportional to the amount of liquid present) However, most solutions (like this one) are non- ideal; in such cases, there can be a positive or negative deviation from Raoult’s Law. - in general, if the two liquids are soluble in each other (like water and acetone), the deviation is negative; if they aren’t, then the deviation is positive - (use intermolecular forces to determine whether they are soluble or not – discussed above) - the pressure exerted by each liquid will be slightly lower (negative deviation) or higher (positive deviation) than expected from Raoult’s Law, which predicts a straight line - the deviation is greatest when the proportions are close to 50%, and none when you have a pure substance (i.e. at the end of the fraction curve) Note that Dalton’s Law still holds; the total pressure will be the sum of the individual partial pressures. However, the total pressure will still deviate, since the individual partial pressures that you are adding have deviated. Now consider graphing the melting temperature as the y- axis on the fraction graph - the two substances do not melt at the same time, and the temperatures depend on the mole fractions – so there will be two curved lines, one above the other - this lines actually represent a phase diagram (and the y- axis is more commonly graphed as simply temperature, which is how it was presented to you) - note that the lines on the phase diagram are not necessarily horizontal, so this means that by moving along the graph horizontally (i.e. changing the mole fraction) can result in phase changes A horizontal line drawn on the graph, from one curve to the other, is a tie line. The tie line associated with any given temperature tells you the mole fractions of the mixture in the liquid phase and solid phase at that temperature. (Tie lines can also be drawn for the vapour pressure...
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