Ch18_Titrations - 1 Titrations 2 Acid-Base Titrations...

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Unformatted text preview: 1 Titrations 2 Acid-Base Titrations Acid-Base Titrations 3 Acid-Base Titrations Acid-Base Titrations pH pH Titrant volume, mL Titrant volume, mL 4 Acid-Base Titrations Acid-Base Titrations 5 QUESTION: You ttitrate100. mL of a QUESTION: You itrate 100. mL of a 0.025 M solution of benzoic acid with 0.025 M solution of benzoic acid with 0.100 M NaOH tto the equivalence point. 0.100 M NaOH o the equivalence point. What is the pH at equivalence point? What is the pH at equivalence point? Equivalence Equivalence point point Additional NaOH is added. pH increases and then levels off as NaOH is added beyond the equivalence point. Additional NaOH is added. pH rises as equivalence point is approached. Adding NaOH from the buret to acetic acid in the flask, a weak acid. In the beginning the pH increases very slowly. Benzoic acid + NaOH NaOH Acid-Base Reactions Section 18.4 QUESTION: You titrate 100. mL of a 0.025 M QUESTION: You titrate 100. mL of solution of benzoic acid with 0.100 M NaOH solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of to the equivalence point. What is the pH of the final solution? the final solution? HBz + NaOH ---> Na+ + Bz-- + H2O HBz NaOH ---> Na + + + H2O pH of solution of pH of solution of benzoic acid, a benzoic acid, a weak acid weak acid C6H5CO2H = HBz HBz Benzoate ion = Bz- 6 Acid-Base Reactions 7 Acid-Base Reactions Section 18.4 QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution? STOICHIOMETRY PORTION STOICHIOMETRY PORTION 1. Calc. moles of NaOH req’d 1. Calc. NaOH (0.100 L HBz)(0.025 M) = 0.0025 mol HBz mol (0.100 L HBz)(0.025 This requires 0.0025 mol NaOH This requires 0.0025 mol 2. Calc. volume of NaOH req’d 2.Calc. NaOH 0.0025 mol (1 L / 0.100 mol) = 0.025 L 0.0025 mol (1 mol) 25 mL of NaOH req’d 25 mL of NaOH Strategy — find the conc. of the find the conc. conjugate base Bz-- iin the solution AFTER conjugate base Bz n the solution AFTER the titration, then calculate pH. the titration, then calculate pH. This is a two-step problem This is a two-step problem Bz-- + H2O ¸ HBz + OH-Bz OH 2 1. stoichiometry of acid-base reaction 1. of acid-base reaction + 2. equilibrium calculation 2. + Kb = 1.6 x 10 -10 10 Acid-Base Reactions QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution? STOICHIOMETRY PORTION STOICHIOMETRY PORTION 25 mL of NaOH req’d 25 mL of NaOH 3. Moles of Bz-- produced = moles HBz = 3. Moles of Bz produced = moles HBz 0.0025 mol 0.0025 mol 4. Calc. conc. of Bz-4. Calc. conc. Bz There are 0.0025 mol of Bz-- iin a TOTAL There are 0.0025 mol of Bz n a TOTAL TOTAL SOLUTION VOLUME of 125 mL SOLUTION VOLUME of [Bz-] = 0.0025 mol / 0.125 L = 0.020 M 0.020 9 Acid-Base Reactions QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution? The product of the titration of benzoic acid, The product of the titration of benzoic acid, the benzoate ion, Bz--,, is the conjugate base the benzoate ion, Bz is the conjugate base of a weak acid. of a weak acid. Therefore, final solution is basic. Therefore, final solution is basic. ¸ 8 11 Acid-Base Reactions Acid-Base Reactions QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH at equivalence point? Equivalence Point Equivalence Point Most important species in solution is benzoate Most important species in solution is benzoate ion, Bz--,, the weak conjugate base of benzoic ion, Bz the weak conjugate base of benzoic acid, HBz. acid, HBz HBz. Bz-- + H2O ¸ HBz + OH-Kb = 1.6 x 10-10 Bz + H2O ¸ HBz OH Kb = 1.6 x 10-10 [Bz--] [HBz] [OH--] [HBz] [OH ] initial 0.020 0 0 0.020 initial 0 0 change -x +x +x +x change -x +x equilib 0.020 - x x x equilib 0.020 - x x x 12 Acid-Base Reactions Acid-Base Reactions QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH at equivalence point? Equivalence Point Equivalence Point Most important species in solution is benzoate ion, Bz--,, Most important species in solution is benzoate ion, Bz Bz the weak conjugate base of benzoic acid, HBz. the weak conjugate base of benzoic acid, HBz HBz. -10 Bz-- + H2O ¸ HBz + OH-Kb = 1.6 x 10-10 Bz + H2O ¸ HBz + OH Kb = 1.6 x 10 Kb 1.6 x 10-10 = x2 0.020 - x x = [OH-] = 1.8 x 10 -6 pOH = 5.75 -----> pH = 8.25 pOH pH 13 13 QUESTION: You ttitrate100. mL of a QUESTION: You itrate 100. mL of a 0.025 M solution of benzoic acid with 0.025 M solution of benzoic acid with 0.100 M NaOH tto the equivalence point. 0.100 M NaOH o the equivalence point. What is the pH at half-way point? What is the pH at half-way point? Half-way Half-way point point 14 Acid-Base Reactions Acid-Base Reactions QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH What is the pH at the half-way point? HBz + H2O ¸ H3O+ + Bz-HBz + H2O ¸ H3O+ + Bz Ka = 6.3 x 10 -5 Ka = 6.3 x 10 -5 Both HBz and Bz-- are present Both HBz and Bz are present [H3 O ] = [HBz] •K a [Bz- ] At the half-way point, [ HBz] = [Bz-] Therefore, [H 3O+] = Ka = 6.3 x 10 -5 pH = 4.20 ...
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This note was uploaded on 10/19/2011 for the course CHM 2210 taught by Professor Reynolds during the Fall '01 term at University of Florida.

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