QCA7e_ch09_szb_handouts

QCA7e_ch09_szb_handouts - Daniel C. Harris Quantitative...

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1 Quantitative Chemical Analysis Seventh Edition Quantitative Chemical Analysis Seventh Edition Chapter 9 Monoprotic Acid-Base Equilibria Daniel C. Harris Daniel C. Harris Chapter Highlights • Strong Acids and Bases – Very dilute solutions • Weak Acids and Bases – Structure and acidity – Calculating the pH • Buffers – Henderson-Hasselbalch equation – Buffer Capacity
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2 Strong Acids and Bases • Calculate the pH of 0.10 M HBr. HBr is a strong acid, so the reaction goes to completion, and the concentration of H 3 O + is 0.10 M. • It will be our custom to write H + instead of H 3 O + , and we can say Strong Acids and Bases • How do we calculate the pH of 0.10 M KOH? • KOH is a strong base (completely dissociated), so [OH ] = 0.10 M. Using K w = [H + ][OH ], we write:
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3 Strong Acids and Bases • How do we calculate the pH of various KOH solutions? Very Dilute Strong Acid/Base • What is the pH of 1.0 10 8 M KOH? • Applying our usual reasoning, we calculate: • But how can the base KOH produce an acidic solution (pH < 7) when dissolved in pure water? It's impossible!
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4 Very Dilute Strong Acid/Base • We have not considered the contribution of OH from the ionization of water. In pure water, [OH ] = 1.0 10 7 M, which is greater than the amount of KOH added to the solution. Very Dilute Strong Acid/Base 1. Pertinent reactions. The only one is 2. Charge balance. The species in solution are K + , OH , and H + . So, 3. Mass balance. All K + comes from the KOH, so [K + ] = 1.0 10 8 M. 4. Equilibrium constant expression. K w = [H + ][OH ] = 1.0 10 14 . 5. Count. There are three equations and three unknowns ([H + ], [OH ], [K + ]), so we have enough information to solve the problem.
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5 Very Dilute Strong Acid/Base 5. Solve. Because we are seeking the pH, let's set [H + ] = x. Writing [K + ] = 1.0 10 8 M we get: Rejecting the negative concentration, we conclude that Strong Acid/Base Conclusions • When the concentration is “high” (> 10 6 M), pH is calculated by just considering the added H + or OH . • When the concentration is “low” (< 10 8 M), the pH is 7.00. We have not added enough acid or base to change the pH of the water itself. • At intermediate concentrations of 10 6 to 10 8 M, the effects of water ionization and the added acid or base are comparable. In this region, a systematic equilibrium calculation is necessary. –C > 1 0 –6 is the only practical case. Unless you were to protect 10 7 M KOH from the air, the pH would be
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6 Weak Acids and Bases • Let's review the meaning of the acid dissociation constant, K a , for the acid HA: – A weak acid is one that is not completely dissociated. That is, the above reaction does not go to completion. • For a base, B, the base hydrolysis constant, K
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QCA7e_ch09_szb_handouts - Daniel C. Harris Quantitative...

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