sinha.FPDepression 3.45.59 PM

sinha.FPDepression 3.45.59 PM - Chemistry 1C Fre ezing...

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Chemistry 1C Freezing Point Depression Larson/Daley 1 May 23, 2008 FPDepression.doc Freezing Point Depression, the van t Hoff Factor, and Molar Mass Objectives To understand colligative properties. To find the freezing point depression of a solution. To determine the van't Hoff factor for acetic acid dissolved in cyclohexane. To deduce the nature of acetic acid dissolved in cyclohexane. To find the molar mass of an unknown organic compound. Discussion – Freezing Point Depression The vapor pressure of a solution containing a non-volatile solute is lower than that of the pure solvent. Consequently, the boiling point of a solution is higher. The freezing point of a solvent is also affected by the solute; the freezing point of a solution is lower than that of the pure solvent. The freezing point of a solution, T f , is T f = T° f T f Where T° f is the freezing point of the pure solvent and T f is the freezing point depression. Freezing point depression, like boiling point elevation, is termed a colligative property. Such properties depend only on the concentration of the solute particles in the solution, and not on their nature. For dilute solutions, T f = mK f = T° f – T f (1) Where m is the molality of the solution and K f is the molal freezing point depression constant, a property of the solvent. Values of K f for various solvents are given in table I. Table I Consider solutions of electrolytes, such as NaCl, dissolved in water; in this case the solute dissociates to form ions in the solution. Thus there are more solute particles in solution than for the same molality of a non-electrolyte, such as sucrose or methanol, dissolved in water. For dilute solutions of NaCl(aq), the solution contains essentially completely separated Na + (aq) and Cl (aq) ions. As a result, the molality of the solute particles (ions in this case) is twice what it would be for a non-electrolyte, and T f is twice as large. On the other hand, if the solute particles associate in solution to form aggregates then the number of solute particles (aggregates in this case) is reduced, causing T f to be smaller than if the solute did not associate. The degree of dissociation or association of the solute particles is given by the van't Hoff factor, i . This factor is the ratio of the number of moles of solute particles in solution to the number of moles of solute dissolved. i = moles solute particles/moles solute dissolved If the solute exists as single molecules (neither dissociating or associating) in solution, then i = 1 If the solute dissociates in solution, then i > 1 If the solute associates in solution, then i < 1 The value of the van't Hoff factor depends upon: the nature of the solute AND solvent and the concentration of the solute in the solution. We can determine the van't Hoff factor experimentally for a given system, that is for a given solute, solvent and concentration of solute, by comparing the experimentally measured freezing
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This note was uploaded on 12/09/2011 for the course CHEM 1 taught by Professor Staff during the Summer '11 term at Simon Fraser.

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sinha.FPDepression 3.45.59 PM - Chemistry 1C Fre ezing...

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