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TitrationsTitrationsTitrationsTitrations
pHpHpHpH
Titrant volume, mLTitrant volume, mLTitrant volume, mLTitrant volume, mLCopyright © 1999 by Harcourt Brace & CompanyAll rights reserved.Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
Acid-Base TitrationsAcid-Base TitrationsAcid-Base TitrationsAcid-Base Titrations
Adding NaOH from the buret to acetic acid in the flask, a weak Adding NaOH from the buret to acetic acid in the flask, a weak acid. In the beginning the pH increases very slowly.acid. In the beginning the pH increases very slowly.
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
Acid-Base TitrationsAcid-Base TitrationsAcid-Base TitrationsAcid-Base Titrations
Additional NaOH is added. pH rises as Additional NaOH is added. pH rises as equivalence point is approached.equivalence point is approached.
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
Acid-Base TitrationsAcid-Base TitrationsAcid-Base TitrationsAcid-Base Titrations
Additional NaOH is added. pH increases and then levels Additional NaOH is added. pH increases and then levels off as NaOH is added beyond the off as NaOH is added beyond the equivalence pointequivalence point..
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
QUESTION:QUESTION: You titrate 100. mL of a 0.025 M solution You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final equivalence point. What is the pH of the final solution?solution?
QUESTION:QUESTION: You titrate 100. mL of a 0.025 M solution You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final equivalence point. What is the pH of the final solution?solution?
Equivalence Equivalence pointpointEquivalence Equivalence pointpoint
pH of solution of pH of solution of benzoic acid, a benzoic acid, a weak acidweak acid
pH of solution of pH of solution of benzoic acid, a benzoic acid, a weak acidweak acid
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
QUESTION:QUESTION: How do we calculate pH at How do we calculate pH at the equivalence point?the equivalence point?
QUESTION:QUESTION: How do we calculate pH at How do we calculate pH at the equivalence point?the equivalence point?
Equivalence Equivalence pointpointEquivalence Equivalence pointpoint
pH of solution of pH of solution of benzoic acid, a benzoic acid, a weak acidweak acid
pH of solution of pH of solution of benzoic acid, a benzoic acid, a weak acidweak acid
QUESTION:QUESTION: How do we How do we calculate calculate pH before pH before titration?titration?
QUESTION:QUESTION: How do we How do we calculate calculate pH before pH before titration?titration?
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
Acid-Base ReactionsAcid-Base ReactionsAcid-Base ReactionsAcid-Base Reactions
HBz reacts completely to produce Bz- HBz reacts completely to produce Bz- Part 1 calculation:Part 1 calculation: stoichiometry gives [Bz stoichiometry gives [Bz--] ]
produced is 0.020 M.produced is 0.020 M.
BzBz- - + H+ H22O O HBz + OH HBz + OH- - KKbb = 1.6 x 10 = 1.6 x 10-10-10
[Bz[Bz--]] [HBz][HBz] [OH[OH--]] equilibequilib 0.020 - x0.020 - x xx xx
x = [OHx = [OH--] = 1.8 x 10] = 1.8 x 10-6-6, pOH = 5.75, and , pOH = 5.75, and pH = 8.25pH = 8.25
HBz reacts completely to produce Bz- HBz reacts completely to produce Bz- Part 1 calculation:Part 1 calculation: stoichiometry gives [Bz stoichiometry gives [Bz--] ]
produced is 0.020 M.produced is 0.020 M.
BzBz- - + H+ H22O O HBz + OH HBz + OH- - KKbb = 1.6 x 10 = 1.6 x 10-10-10
[Bz[Bz--]] [HBz][HBz] [OH[OH--]] equilibequilib 0.020 - x0.020 - x xx xx
x = [OHx = [OH--] = 1.8 x 10] = 1.8 x 10-6-6, pOH = 5.75, and , pOH = 5.75, and pH = 8.25pH = 8.25
QUESTION:QUESTION: You titrate 100. mL of a 0.025 M solution of You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution?What is the pH of the final solution?
Kb 1.6 x 10-10 = x2
0.020 - xKb 1.6 x 10-10 =
x2
0.020 - x
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
Half-way Half-way pointpointHalf-way Half-way pointpoint
[HBz] = [Bz[HBz] = [Bz--]]
At midpoint of titration…At midpoint of titration…
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
Acid-Base ReactionsAcid-Base ReactionsAcid-Base ReactionsAcid-Base Reactions
HBzHBz + H+ H22O O H H33OO++ + Bz + Bz- - KKaa = 6.3 x 10 = 6.3 x 10-5-5
pH = pKa + log ( [BzpH = pKa + log ( [Bz- - ]/[HBz] )]/[HBz] )
pH = 4.20 + log (1)pH = 4.20 + log (1)
pH = 4.20pH = 4.20
HBzHBz + H+ H22O O H H33OO++ + Bz + Bz- - KKaa = 6.3 x 10 = 6.3 x 10-5-5
pH = pKa + log ( [BzpH = pKa + log ( [Bz- - ]/[HBz] )]/[HBz] )
pH = 4.20 + log (1)pH = 4.20 + log (1)
pH = 4.20pH = 4.20
QUESTION:QUESTION: You titrate 100. mL of a 0.025 M You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH solution of benzoic acid with 0.100 M NaOH What is the pH at the half-way point?What is the pH at the half-way point?
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
Half-way Half-way pointpointHalf-way Half-way pointpoint
……can calculate pH if know [Bzcan calculate pH if know [Bz- - ]/[HBz] ]/[HBz]
At any point of the titration…At any point of the titration…
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Copyright (c) 1999 by Harcourt Brace & CompanyAll rights reserved
pH = pKapH = pKapH = pKapH = pKa
……can calculate pH if know [Bzcan calculate pH if know [Bz- - ]/[HBz] ]/[HBz]
At any point of the titration…At any point of the titration…
If pH > pKa, If pH > pKa, [Bz[Bz- - ]>[HBz]]>[HBz]If pH < pKa, If pH < pKa,
[HBz]>[Bz[HBz]>[Bz--]]