Chem 287_Ch.11

Chem 287_Ch.11 - Lectures: Classical Methods of Chemical...

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Lectures: Classical Methods of Chemical Analysis 1 Polyprotic Acid-Bases and Titrations To be covered: Chapter 11 Problems: 2,9,11,16,28 1) Polyprotic Acids 2) Fractional Composition 3) Polyprotic Weak acid-strong base titration Polyprotic Acids Polyprotic acid can be treated in a similar manner as monoprotic acid buffers and titrations as monoprotic acid buffers and titrations. Example: CO 2 in Water CO 2 + H 2 O ֕ H 2 CO 3 ֕ H + + HCO 3 - pK a1 = 6.35 HCO 3 - ֕ H + + CO 3 -2 pK a2 = 10.33 Pure water in air has a pH = 5.65. What is the molar ratio of [CO 2 ] / [HCO 3 - ]? pH = 5.65 = 6.35 + Log ( [HCO 3 - ] / [CO 2 ] ) ratio = 0.200
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Lectures: Classical Methods of Chemical Analysis 2 Polyprotic Acid-Example In natural fresh bodies of water pH is different than in pure solution. This is due to the CaCO 3 container. Determine the pH for a CO 2 = 1.146 x 10 -5 M and CO 3 -2 = 8.96 x 10 -6 M for the reaction: CO 2 + H 2 O ֕ H + + HCO 3 - pK a1 = 6.35 HCO 3 - ֕ H + +C O 3 -2 pK 2 = 10.33 + CO a2 10.33 pH = 1/2 ((pK a1 + pK a2 ) +Log( [CO 3 -2 ] / [CO 2 ])) = 1/2 ((16.68) + (-0.1069)) = 8.29 Fractional Composition Since dissociation of the acid is related to pH. It is ft
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This note was uploaded on 07/15/2011 for the course CHEM 287 taught by Professor Burns during the Fall '08 term at McGill.

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Chem 287_Ch.11 - Lectures: Classical Methods of Chemical...

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