CCSF CHEM 101B - FREEZING POINT DEPRESSION (3 pages)

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FREEZING POINT DEPRESSION



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FREEZING POINT DEPRESSION

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Pages:
3
School:
City College of San Francisco
Course:
Chem 101b - General College Chem - Lecture
General College Chem - Lecture Documents

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FREEZING POINT DEPRESSION DISCUSSION 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 depression Tf is defined as Tf freezing point of solvent freezing point of solution The freezing point depression like the boiling point elevation is termed a colligative property Such properties depend only on the concentration of the solute particles and not on their nature For dilute solutions Tf mKf where m molality moles of soute particles per kg of solvent Kf molal freezing point depression constant a property of the solvent For solutions of electrolytes such as NaCl the solute ionizes in solution Thus there are more solute particles than for the same number of moles of a nonelectrolyte For NaCl Na aq and Cl aq exist in solution As a result the molality of the solute particles ions in this case is twice that for a nonelectrolyte and Tf is twice as large However if the solute particles associate in solution to form aggregates then the number of solute particles aggregates in this case is reduced causing Tf 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 If the solute dissociates in solution i 1 If the solute associates in solution i 1 An alternative form of the equation above that explicitly takes into account dissociation or association of the solute is shown below Tf imKf m molality of solute without taking dissociation or association into consideration In this experiment you are going to investigate the nature of acetic acid dissolved in a non polar solvent cyclohexane OBJECTIVES 1 2 3 4 To



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