Buffers and K a ’s

What are Buffers? At times, it is essential to maintain a certain pH. At times, it is essential to maintain a certain pH. To achieve this, buffers or buffer solutions are used. To achieve this, buffers or buffer solutions are used. A solution made from a weak acid and its conjugate base that neutralizes small amounts of acids or bases added to it. A solution made from a weak acid and its conjugate base that neutralizes small amounts of acids or bases added to it. It relies on Le Châtelier's principle to “absorb” the excess OH - or H + ions It relies on Le Châtelier's principle to “absorb” the excess OH - or H + ions

Buffers are Everywhere Buffers act as preservatives, since pH affects taste. Buffers act as preservatives, since pH affects taste. Phosphoric acid is added to food to buffer the pH to help maintain the taste. Phosphoric acid is added to food to buffer the pH to help maintain the taste. Blood has a buffer system. Blood has a buffer system. If the pH of blood varies to far from 7.4, sickness or even death can occur. If the pH of blood varies to far from 7.4, sickness or even death can occur.

Weak Acids and Bases Weak acids and bases only partially ionize. Therefore, they reach equilibrium. Weak acids and bases only partially ionize. Therefore, they reach equilibrium. Since they reach equilibrium, they have equilibrium constants. Since they reach equilibrium, they have equilibrium constants. These constants are called acid-ionization constants and are abbreviated K a These constants are called acid-ionization constants and are abbreviated K a As before, products go on top, reactants go on bottom, and solids and liquids are ignored. As before, products go on top, reactants go on bottom, and solids and liquids are ignored. Table 7 on page 559 lists some weak acids and their conjugate bases Table 7 on page 559 lists some weak acids and their conjugate bases

Helpful Hints Weak acids will lose one H + at a time and each loss will have a different K a constant Weak acids will lose one H + at a time and each loss will have a different K a constant This means that the [H 3 O + ] and the other product have the same concentration. This means that the [H 3 O + ] and the other product have the same concentration. It also means that the coefficients are one for everything in the equation. It also means that the coefficients are one for everything in the equation. Sometimes, the pH will be given, which will need to be changed to [H 3 O + ] first. Sometimes, the pH will be given, which will need to be changed to [H 3 O + ] first.

Example Problems Calculate [H 3 O + ] of a M acetic acid solution. (K a on page 559 is 1.75 x ) Calculate [H 3 O + ] of a M acetic acid solution. (K a on page 559 is 1.75 x ) Find K a if a 0.50 M solution of a weak acid has a hydronium ion concentration of 1.3 x Find K a if a 0.50 M solution of a weak acid has a hydronium ion concentration of 1.3 x A solution prepared by dissolving 1.0 mol of benzoic acid in water to form 1.0 L of solution has a pH of 2.1. Calculate the acid-ionization constant. A solution prepared by dissolving 1.0 mol of benzoic acid in water to form 1.0 L of solution has a pH of 2.1. Calculate the acid-ionization constant.