Weak Acid - ² pKa = 5.4 ² Phenolphthalein as indicator...

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CHEM 205 Lecture #9 Dr. Nicholas Llewellyn Characterization of a Weak Acid
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Brønsted-Lowry Theory ± Acid: Proton donor ± Base: Proton acceptor HA + B ² A ² + HB Acid + Base Conjugate Base + Conjugate Acid
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Water ± Amphiprotic ² Can accept or donate H + ² HA + H 2 O A ² + H 3 O + ² H 2 O + B ² OH ² + HB ± Water may act as an acid or a base ² H 2 O + H 2 O H 3 O + + OH ² ² Kw = [ H 3 O + ][ OH ² ] = 1.0 º 10 -14
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Acid Strength ± Degree of dissociation: ² HA H + + A ² ² K a = ; pK a = -log(K a ) ± Acid strength relative to H 3 O + (pK a = -1.7) ² Strong acid has pK a < -1.7 ² Weak acid has pK a > -1.7 [ H + ][ A ² ] [ HA ]
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This Week ± Characterize and identify an unknown weak acid by ³DXWRPDWHG´ titration. ² Molar mass ² pK a
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Titration ± Chemical vocabulary term ² To quantify something by adding something else to it slowly and carefully until stuff happens. ± Not specific to any particular analyte or method of detection ² Redox Titration ² Complexometric Titration
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± Already done in Cobalt labs ² Ammonia Ligands and Charge ± 6WDQGDUGL]H \RXU ³µ¶· 0´ NaOH against a carefully weighed quantity of KHP ² Potassium Hydrogen Phthalate
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Unformatted text preview: ² pKa = 5.4 ² Phenolphthalein as indicator ³$XWRPDWHG´ 7LWUDWLRQ ± Use a drop counter and pH meter ² 'RQ¶W XVH DQ indicator ² Careful when starting flow of titrant Weak Acid Titration Curve ± Molar mass determined from mass of unknown and number of moles NaOH consumed at equivalence point. Weak Acid Titration Curve ± K a determined from pH at or near the half-equivalence point ² ³%XIIHULQJ UHJLRQ´ Determining K a ± At the half-equivalence point: ² K a = ² [ A ² ] = [ HA ] ; = 1 ² K a = [ H + ] [ H + ][ A ² ] [ HA ] [ A ² ] [ HA ] ( ) Determining K a ± Near the half-equivalence point: ² K a = ; pK a = -log ² pK a = pH ² log ² pH = pK a + log ² Henderson-Hasselbalch [ H + ][ A ² ] [ HA ] [ H + ][ A ² ] [ HA ] ( ) [ A ² ] [ HA ] [ A ² ] [ HA ] ( ) Diprotic Acids ± Two K a s: K a1 and K a2 . Identifying Your Unknown ± Molar Mass + pK a + Merck/CRC...
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This note was uploaded on 11/19/2010 for the course CHEM 204 taught by Professor Gruebele during the Spring '08 term at University of Illinois, Urbana Champaign.

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Weak Acid - ² pKa = 5.4 ² Phenolphthalein as indicator...

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