Acids_V_Titrations (2)

Acids_V_Titrations (2) - TITRATIONS Equivalence Point...

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TITRATIONS Equivalence Point Acid-Base Indicator End Point McQuarrie
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TITRATIONS McQuarrie
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TITRATION OF STRONG ACID WITH STRONG BASE McQuarrie
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E11 TITRATION OF WEAK ACID WITH STRONG BASE pH at midpoint? Note, the equivalence point is basic (strong base beats weak acid of = conc.) [] 0 0 log aa B pH pK pK HB ⎛⎞ ⎡⎤ ⎣⎦ ⎜⎟ =+ = ⎝⎠ McQuarrie
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TITRATION OF WEAK BASE WITH STRONG ACID Note, the equivalence point is acidic (strong acid beats weak base of = conc.) pH at midpoint? [] 0 0 log aa B pH pK pK HB ⎛⎞ ⎡⎤ ⎣⎦ ⎜⎟ =+ = ⎝⎠ awb where pK pK pK =− McQuarrie
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INDICATORS pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Thymol Blue McQuarrie
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ACID-BASE INDICATORS ARE BUFFERS Acid Conjugate Base Low or high pH yellow? Hint—Le Chatelier’s Principle Low High [] log when ai ai In pH pK pK HIn In HIn ⎛⎞ ⎡⎤ ⎣⎦ ⎜⎟ =+ = = ⎝⎠ Color? Green Turns out indicator is green at pH = 7. What is K ai ? Bromthymol Blue K ai 10 -7 How do you pick an indicator? Want pH equivalence point pK ai McQuarrie
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McQuarrie
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SOME INDICATORS A colorless solution is blue when bromcresol green is added and red when methyl red is added. Approx. pH? A colorless solution is pink when phenolphthalein is added and colorless when thymolphthalein is added. Approx. pH? McQuarrie
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TITRATION OF STRONG ACID WITH STRONG BASE McQuarrie
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TITRATION OF STRONG ACID WITH STRONG BASE EXAMPLE 23: 40mL of 0.15 M HBr is titrated with 0.12 M KOH. What is the pH of the acid after (a) 0, (b) 10, (c) 30, and (d) 60 mL of the base have been added? (e) How many mL have KOH have been added at the equivalence point and what is the pH ? (a) pH = -log[ H + ] = -log(0.15) = 0.82 (b) Find mmol (or mol) for initial concentrations: () 0.15 40 6 mol mmol H mL mmol L + == 10 L 0.12 mol 1.2 mmol OH mL mmol ( ) ( ) ( ) 2 Ha q O q H O l +− +→ Init. 6 mmol 1.2 mmol -1.2 Change 4.8 0 Eqm. -1.2 Calculate the molarity: 4.8 0.0960 50 mmol H M mL + ⎡⎤ ⎣⎦ Setup ICE table: pH = -log(0.096) = 1.02
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(c) Find mmol (or mol) for initial concentrations: 6 mmol H mmol + = () 0.12 30 3.6 mol mmol OH mL mmol L == ( ) ( ) ( ) 2 Ha q O q H O l +− +→ Setup ICE table: Init. 6 mmol 3.6 mmol -3.6 -3.6 Change 2.4 0 Eqm. Calculate the molarity: 2.4 0.0343 70 mmol H M mL + ⎡⎤ ⎣⎦ pH = -log(0.0343) = 1.46
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(d) Find mmol (or mol) for initial concentrations: 6 mmol H mmol + = () 0.12 60 7.2 mol mmol OH mL mmol L == ( ) ( ) ( ) 2 Ha +− Init. 6 mmol q O q H O l +→ Setup ICE table: Change Eqm. 7.2 mmol 0 1.2 -6 -6 Calculate the molarity: 1.2 0.0120 100 mmol OH M mL ⎡⎤ ⎣⎦ pOH = -log(0.012) = 1.92 pH = 14 - 1.92 = 12.08
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(e) 6 50 10 . 1 2 mmol OH mL xm L O H mmol = 0.12 or 6 mmol XmL mmol OH mL ⋅= At the equivalence point between a strong acid and a strong base, the pH is 7 since the aqueous salt (KBr in this example) consists of neutral ions, that is, the only source of H + and OH - is water.
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This note was uploaded on 03/25/2011 for the course CHEMISTRY 126 taught by Professor Ellis during the Spring '11 term at NJIT.

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Acids_V_Titrations (2) - TITRATIONS Equivalence Point...

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