ln4s08 - Colligative Properties Page 1 Lecture 4...

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Colligative Properties- Page 1 Lecture 4: Colligative Properties ! By definition a colligative property is a solution property (a property of mixtures) for which it is the amount of solute dissolved in the solvent matters but the kind of solute does not matter. ! Coming to grips with this concept should immediately remind you of kinetic molecular theory of gases—in that case we treated gas molecules as indistinguishable hard spheres and (ideally) it was the number of them, rather than the type of molecules, that determined gas properties. ! This means that when considering the impact of solute on a colligative property, 1 mole of sugar " 1 mole Na # " 1 mole O $ 2 " 1 mole urea " 1 mole pickles do exactly the same thing Listed below are the four colligative properties we will examine during this lecture—each is kind of fun because it is associated with fairly famous physical phenomena that you might like to explain to a friend . Colligative Properties For each of these properties you will be introduced to the physical phenomenon behind the property and learn how to perform simple calculations to determine the magnitude the change in solution state function associated with a colligative property. % Vapor Pressure Lowering—explains the value of putting antifreeze in your radiator to keep a car from overheating % Boiling Point Raising—explains how you can cook spaghetti faster in salt water % Freezing Point Lowering explains why salt is placed on roads to keep ice from forming % Osmotic Pressure explains why your little brother killed the family fish when he placed them in pure water while cleaning the tank.
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Colligative Properties- Page 2 Concentration and Colligative Properties If colligative properties depend on the amount of the solute in the solvent, then the equations defining them must include a concentration term, and sure enough, they do. Over the next few pages you will be introduced to the equations in the context of the specific properties, but for now, simply note the similarities in structure for the equations: each equation includes a colligative property on the left side of the equation that is set equal to a concentration term and a solvent constant. P vapor pressure T temperature raising or lowering osmotic pressure T f = - m K f P = ( P 0 T f = - m K f = M RT Three properties set equal to three different concentrations terms times a solvent constant Let’s practice performing concentration calculations. Two of these, molarity, M, and mole fraction, X, should be familiar to you. A third, molality, m, may be new. But all are useful ways to define the amount of stuff in solution—the more stuff, the larger the concentration. Let’s start by imagining that we are placing 50 g (0.146 mole) of the solute, sugar, in 117 g (6.5 mole) of the solvent, H
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ln4s08 - Colligative Properties Page 1 Lecture 4...

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