CH131_11B_12A

# CH131_11B_12A - Colligative Properties of Solutions These...

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Colligative Properties of Solutions These are properties of solutions that just depend on the concentration of solute particles; not on their chemical identities. 1. Vapor pressure lowering P 1 Pure liquid Solution (nonvolatile solute) No v.p. itself P 2 Observed: vapor pressure of pure liquid (P 1 ) > vapor pressure of solution (P 2 ) For many solutions the v.p. of solution given by: => Raoult’s Law where A is the solvent: x A mole fraction of A; P a o = vapor pressure of pure A; P A is v.p. of A with a nonvolatile solute in it P A = x A P A o

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Remember the phase diagram for a pure substance (plot of P vs. T showing phases and equilibria) Suppose we put a nonvolatile solute in liquid (binary system). What happens to the phase diagram? By Raoult’s Law: or at a given T, so on the phase diagram v. p. reduced at each T P A = x A P A o P = x solute P A o 1 atm T b T b P T b = T b - T b Consequence, normal boiling point pushed higher and freezing point pushed lower! T b = T b - T b boiling point elevation T f = T f - T f freezing point depression T f ’T f
More quantitatively,… Since v.p. lowering amount of solute dissolved, we can expect T b ( T f ) to be amount of solute dissolved P = x solute P A o For a dilute solution x solute m (molality) i.e. x solute = n solute n solvent + n solute n solute n solvent For dilute solution n solvent >> n solute But so n solvent = mass ( kg ) solvent M ( kg / mol ) x solute n solute mass ( kg ) solvent = m Thus, we write m = amount of solute dissolved (molality) K b , K f are constants that depend on solvent (not solute) T b = K b m D T f = - K f m (increase) (decrease) The identity of solute not important: only the amount of species dissolved determines the b.p. elevation & f.p. depression

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Osmotic Pressure pure solution P o > P > x solve P o Consider this situation: 2 beakers, (1) pure solvent, (2) solution v.p. different over the 2 liquids not at equil. Both liqs. in contact w/ same vapor at some P, but not the P required for equilibrium. So to establish equil. value of P a. Liq. evap. from pure beaker to try to establish P o , i.e. make P larger b. Vap condenses above soln. to try to reduce P, i.e. bring it closer to x solvent P o Result=> net transfer of solvent from pure beaker to solution beaker! Equilibrium (no change evident) attained when all liq. transfered to solution beaker
Driving force => vapor pressure difference (due to nonvolatile solute) and air permeable to solvent only . What’s this got to do with colligative properties? Another way of viewing the same phenomenon is to have a solution

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CH131_11B_12A - Colligative Properties of Solutions These...

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