   Chapter 15, Problem 4RQ

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Textbook Problem

# A good buffer generally contains relatively equal concentrations of weak acid and conjugate base. If you wanted to buffer a solution at pH = 4.00 or pH = 10.00, how would you decide which weak acid–conjugate base or weak base–conjugate acid pair to use? The second characteristic of a good buffer is good buffering capacity. What is the capacity of a buffer? How do the following buffers differ in capacity? How do they differ in pH?0.01 M acetic acid/0.01 M sodium acetate0.1 M acetic acid/0.1 M sodium acetate1.0 M acetic acid/1.0 M sodium acetate

Interpretation Introduction

Interpretation: The behavior of a buffer that has equal concentration of weak acid-conjugate base and good buffering capacity is given. The weak acid-conjugate base or weak base-conjugate base chosen for the preparation of buffer having pH=4.0 or pH=10.0 ; the concept of capacity of buffer; the difference in buffer capacity of given solutions and their pH is to be compared.

Concept introduction: A strong acid has a weak conjugate base while a weak acid has strong conjugate base and similar is the case for a base. A strong base has weak conjugate acid while a weak base has a strong conjugate acid.

To determine: The weak acid-conjugate base or weak base-conjugate base chosen for the preparation of buffer having pH=4.0 or pH=10.0 ; the concept of capacity of buffer; the difference in buffer capacity of given solutions and their pH .

Explanation

Explanation

While designing a buffer solution, the weak acid and conjugate base is chose such that the pH of the solution is almost near to pKa . It means the concentration of weak acid and conjugate base is almost equal. Similarly, such type of weak base and conjugate is chosen whose required pH value is almost near to pKb .

The Henderson-Hasselbalch equation is used for designing a buffer solution. The Henderson-Hasselbalch equation for a weak acid and its conjugate base is represented as shown below.

pH=pKa+log[A][HA]

Where,

• pH is the negative logarithm of H+ ions concentration in the solution.
• pKa is the negative logarithm of dissociation constant of the acid.
• [A] is the concentration of conjugate base of the given weak acid.
• [HA] is the concentration of given weak acid.

While designing a buffer solution, the weak acid and conjugate base is chose such that the pH of the solution is almost near to pKa . It means the concentration of weak acid and conjugate base is almost equal. Similarly, such type of weak base and conjugate is chosen whose required pH value is almost near to pKb .

The Henderson-Hasselbalch equation for a weak base and its conjugate acid is represented as shown below.

pOH=pKb+log[A][BOH]

Where,

• pOH is the negative logarithm of OH ions concentration in the solution.
• pKb is the negative logarithm of dissociation constant of the base.
• [A] is the concentration of conjugate base of the given weak base.
• [BOH] is the concentration of given weak acid.

Explanation

The buffer capacity is measurement of ability of a buffer to resist change in pH .

Those solutions which upon addition of an acid or base resist the change in pH are called buffer solution. The pH of the buffer solution remains constant. The buffer capacity is measurement of ability of a buffer to resist change in pH . It is basically the addition of gram equivalents of an acid or a base to a 1.0L solution so that the pH of the solution changes by 1 unit. The buffer capacity for a solution is given as,

β=ΔBΔpH

Where,

• ΔB is the gram equivalents of acid or base added.
• ΔpH is the change in pH caused by the addition of strong acid or base.

Explanation

The quantities of buffering components is high in third buffer solution, therefore it has the highest buffer capacity, then comes the second buffer solution and the first buffer solution has the least buffer capacity among the three given buffer solutions.

The buffer capacity of a buffer is maximum when the ratio of [C2H3O2][HC2H3O2] is near to 1

Substitute the value of both salt and acid for each part in the above ratio as,

• The buffer capacity for HC2H3O2=0

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