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SGCh16 - Chapter Sixteen ACID-BASE EQUILIBRIA AND...

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329 Chapter Sixteen ACID-BASE EQUILIBRIA AND SOLUBILITY EQUILIBRIA Buffer Solutions Titration Curves and Indicators Solubility and Solubility Product Predicting Precipitation Reactions and Separation of Ions by Precipitation Factors Affecting Solubility: Common Ion Effect and pH Complex Ions and Solubility BUFFER SOLUTIONS STUDY OBJECTIVES 1. Describe the effect of common ions on the percent ionization of weak acids and bases. 2. Calculate the pH of a buffer solution, given the concentrations of weak acid or base and their salts. 3. Determine the pH of a buffer solution after the addition of a small amounts of strong acid base. 4. Describe how to prepare a buffer solution with a specific pH. The Common Ion Effect. A 1.0 M HF solution has equal concentrations of hydrogen ion and fluoride ion; [H + ] = [F ] = 2.6 × 10 –2 M. Upon dissolving enough sodium fluoride (NaF) to bring the F ion concentration up to 1.0 M, the hydrogen ion concentration will fall to 7.1 × 10 –4 M. The effect of addition of NaF to an HF solution is an example of the common ion effect. In this equilibrium, fluoride ion is the common ion. The equilibrium in the original hydrofluoric acid solution was HF(aq) H + (aq) + F (aq) When sodium fluoride, a strong electrolyte, was added the concentration of F ion increased. NaF(aq) Na + (aq) + F (aq) According to Le Chatelier's principle, the addition of F ions will shift the weak acid equilibrium to the left; this consumes some of the F ions and some H + (aq), and lowers the percent ionization. In effect, the percent ionization of a weak acid (HA) is repressed by the addition to the solution of its conjugate base, F ion. The shift in equilibrium caused by the addition of an ion common to one of the products of the original equilibrium reaction is called the common ion effect . Buffer Solutions. Any solution like the preceding one, which contains both a weak acid (HF) and its conjugate base (F ) has the ability to neutralize added strong acids and bases with very little change in its pH. Such a solution is called a buffer solution because it resists significant changes in the pH. A buffer must contain an acid to react with any OH ions that may be added, and a base to react with any added H + ions. A
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330 / Acid-Base Equilibria and Solubility Equilibria solution containing (1) a weak acid or weak base and (2) its salt is a buffer solution. HF(aq) H + (aq) + F (aq) Added substances that form the buffer solution When a small amount of a strong acid is added to the buffer, it is neutralized by the weak base. Also, when a small amount of a strong base is added to a buffer, it is neutralized by the weak acid. In the hydrofluoric acid- fluoride ion buffer, for example, the following neutralization reactions take place: • On addition of a strong base, the neutralization reaction is OH + HF H 2 O + F • On addition of a strong acid, the neutralization reaction is H + + F HF
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