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Tutorial 8 - Solutions

# Tutorial 8 - Solutions - CHEMISTRY 1AA3 March 8-12 2010...

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CHEMISTRY 1AA3 March 8-12, 2010 TUTORIAL PROBLEM SET 8 – Full SOLUTIONS ______________________________________________________________________________ 1. (a) Para-aminobenzoic acid (NH 2 C 6 H 4 COOH, or HPABA for short) is one of the ingredients sometimes found in sunblock lotions. What is the pH of a 0.200 M solution of HPABA? ( K a (HPABA) = 1.2 × 10 5 ). (b) If you titrate 100 mL of the above solution with 0.200 M KOH, what is the pH of the solution: (i) after the addition of 25 mL of the 0.200 M KOH? (ii) after the addition of 50 mL of the 0.200 M KOH? (iii) at the equivalence point? (c) The effective pH range of three indicators is given below. Which of these indicators would be most appropriate for the above titration? Briefly explain why. Methyl Orange pH 4.2 - 6.3 Phenol Red pH 6.8 - 8.2 Phenolphthalein pH 8.0 - 10.0 SOLUTION: Part (a) HPABA + H 2 O PABA + H 3 O + Initial 0.200 M 0 0 Change -x +x +x Equilibrium 0.200-x x x K a = [PABA ][H 3 O + ] = (x)(x) = 1.2 × 10 5 [HPABA] (0.200-x) Assuming x << 0.200, solve for x: x = 1.55 × 10 3 M = [H 3 O + ] and the assumption is valid (0.8 %) pH = log 10 [H 3 O + ] = 2.81 (b) If you titrate 100 mL of the above solution with 0.200 M KOH, what is the pH of the solution: (i) after the addition of 25 mL of the 0.200 M KOH?

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CHEMISTRY 1AA3 March 8-12, 2010 TUTORIAL PROBLEM SET 8 – Full SOLUTIONS ______________________________________________________________________________ Page 2 of 7 SOLUTION: Titration with KOH will consume some of the acid HPABA, and produce some of its conjugate base PABA : HPABA + OH PABA + H 2 O We start with: mol HPABA in 100 mL = (0.200 M)(0.100 L) = 0.0200 mol mol KOH added = (0.200 M)(0.025 L) = 0.005 mol We end up with: mol HPABA after reaction = 0.0200 - 0.005 = 0.0150 mol in 125 mL and mol PABA produced = 0.005 mol in 125 mL Both of these species are now in a total solution volume of 100 mL + 25 mL = 125 mL, and so we will have new concentrations to put into our ICE table: HPABA + H 2 O PABA + H 3 O + Initial 0.120 M 0.0400 M 0 Change -x +x +x Equilibrium 0.120-x 0.0400 + x
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Tutorial 8 - Solutions - CHEMISTRY 1AA3 March 8-12 2010...

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