Buffers and KBuffers and Kaa’s’s

What are Buffers?What are Buffers?

At times, it is essential to maintain a At times, it is essential to maintain a certain certain pHpH..

To achieve this, To achieve this, buffersbuffers or or bufferbuffer solutionssolutions are used. are used.

A solution made from a A solution made from a weakweak acid and acid and its its conjugateconjugate base that base that neutralizesneutralizes smallsmall amounts of acids or bases added amounts of acids or bases added to it.to it.

It relies on It relies on Le Châtelier'sLe Châtelier's principle to principle to ““absorbabsorb” the excess OH” the excess OH-- or H or H++ ions ions

Buffers are Buffers are EverywhereEverywhere Buffers act as Buffers act as preservativespreservatives, since , since

pH affects taste.pH affects taste. PhosphoricPhosphoric acid is added to food to acid is added to food to

buffer the pH to help maintain the buffer the pH to help maintain the taste.taste.

BloodBlood has a buffer system. has a buffer system. If the pH of blood varies to far from If the pH of blood varies to far from

7.47.4, sickness or even death can , sickness or even death can occur.occur.

Weak Acids and BasesWeak Acids and Bases WeakWeak acids and bases only acids and bases only partiallypartially

ionize. Therefore, they reach ionize. Therefore, they reach equilibriumequilibrium.. Since they reach Since they reach equilibriumequilibrium, they have , they have

equilibriumequilibrium constantsconstants.. These constants are called These constants are called acid-ionizationacid-ionization

constantsconstants and are abbreviated and are abbreviated KKaa

As before, As before, productsproducts go on top, go on top, reactantsreactants go on bottom, and go on bottom, and solidssolids and and liquidsliquids are are ignored.ignored.

Table Table 77 on page on page 559559 lists some weak lists some weak acids and their conjugate basesacids and their conjugate bases

Helpful HintsHelpful Hints

WeakWeak acids will lose acids will lose oneone H H++ at a time and at a time and each each lossloss will have a will have a differentdifferent K Kaa constant constant

This means that the This means that the [H[H33OO++]] and the other and the other productproduct have the same concentration. have the same concentration.

It also means that the It also means that the coefficientscoefficients are are oneone for everything in the equation. for everything in the equation.

Sometimes, the Sometimes, the pHpH will be given, which will be given, which will need to be changed to will need to be changed to [H[H33OO++]] first. first.

Example ProblemsExample Problems

Calculate [HCalculate [H33OO++] of a 0.150 M acetic ] of a 0.150 M acetic acid solution. (Kacid solution. (Kaa on page 559 is on page 559 is 1.75 x 101.75 x 10-5-5))

Find KFind Kaa if a 0.50 M solution of a weak if a 0.50 M solution of a weak acid has a hydronium ion acid has a hydronium ion concentration of 1.3 x 10concentration of 1.3 x 10-4-4..

A solution prepared by dissolving 1.0 A solution prepared by dissolving 1.0 mol of benzoic acid in water to form mol of benzoic acid in water to form 1.0 L of solution has a pH of 2.1. 1.0 L of solution has a pH of 2.1. Calculate the acid-ionization Calculate the acid-ionization constant.constant.