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Tutorial 8

# Tutorial 8 - Phenolphthalein pH 8.0 10.0 2 Which of the...

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CHEMISTRY 1AA3 March 8-March 12, 2010 TUTORIAL PROBLEM SET 8 ______________________________________________________________________________ 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
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Unformatted text preview: Phenolphthalein pH 8.0 - 10.0 2. Which of the three acids given below would be appropriate to prepare a buffer solution to maintain the pH at 4.30? What ratio of the acid/conjugate base should be used to fix the pH at 4.30? Acid Conjugate Base K a HSO 4-SO 4 2 − 1.2 × 10 − 2 CH 3 COOH CH 3 COO − 1.8 × 10 − 5 HCN CN − 4.0 × 10 − 10 3. Calculate the pH of the equivalence point when 25 mL of 0.10 M NH 3 is titrated by a 0.10 M HCl solution ( K b for NH 3 = 1.8 × 10 − 5 ). 4. Stinker. Hydrogen sulfide (hydrosulfuric acid) H 2 S is a weak diprotic acid. Calculate the concentrations of the two conjugate bases of a 0.1M solution of the acid in water. K a1 = 1.0 × 10-7 and K a2 = 1.3 × 10-13 . 5. Take up questions from test 2....
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