Hwk 6 Holcombe

# 1 4 10 1 m 2 2 10 1 m 3 1 2 10 2 m correct 4 4 10 2 m

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1. 4 . 0 × 10 - 1 M 2. 2 . 0 × 10 - 1 M 3. 1 . 2 × 10 - 2 M correct 4. 4 . 0 × 10 - 2 M 5. 1 . 0 × 10 - 7 M Explanation: 012 10.0points A 0.15 M solution of a weak triprotic acid is adjusted (base added) so that the pH is 9.27 (you can assume minimal volume change). The triprotic acid (H 3 A) has the following acid ionization constants: K a1 = 1 . 0 × 10 - 3 , K a2 = 1 . 0 × 10 - 8 , K a3 = 1 . 0 × 10 - 12 . Which species given below is present in the highest concentration in this pH 9.27 solution? 1. H 3 O + 2. H 3 A 3. A 3 - 4. HA 2 - correct 5. OH - 6. H 2 A - Explanation: Any pH between 8 and 12 will have the monoprotic species as the major species. 013 10.0points What would be the pH of an aqueous solu- tion of NH 4 Br? 1. pH > 7 2. pH < 7 correct 3. pH = 7 Explanation: This is a solution of the salt of a strong acid and a weak base. It consists therefore, of the cation (the con- jugate base of the strong acid) which is itself extermely weak and has no effect on pH and the anion (the conjugate acid of the weak base) which is able to partly deprotonate. Thus the solution is acidic. NH + 4 (aq) + H 2 O( ) H 3 O + (aq) + NH 3 (aq) pH < 7 014 10.0points What would be the pH of a 7 . 0 × 10 - 8 M solution of HNO 3 ? 1. 7.013 2. 7.155 3. 6.987 4. 5.973 5. 6.770 6. 6.851 correct Explanation: To solve for this dilute solution you must use (and solve) the charge balance equation: [H + ] = [OH - ] + [NO - 3 ] The concentration for NO - 3 is 7 . 0 × 10 - 8 M. Now solving for [H + ] [H + ] 2 - 7 × 10 - 8 [H + ] - K w = 0 [H + ] = x = parenleftBigg - b ± b 2 - 4ac 2a parenrightBigg [H + ] = 1 . 409 × 10 - 7 M pH = 6.851 015 10.0points Write the charge balance for a solution that initially contains CsF and CaCO 3 .

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• Fall '07
• Holcombe
• pH, Acids

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