   Chapter 17, Problem 18PS

Chapter
Section
Textbook Problem

Which of the following combinations would be the best to buffer the pH of a solution at approximately 7? (a) H3PO4 and NaH2PO4 (b) NaH2PO4 and Na2HPO4 (c) Na2HPO4 and Na3PO4

a)

Interpretation Introduction

Interpretation:

A buffer solution is to be prepared with pH value approximately 7.0. The following pair is suitable, H3PO4andNaH2PO4 has to be predicted.

Concept introduction:

The Henderson-Hasselbalch equation relates pH of a buffer with pKa of acid, concentration of conjugate base and concentration of acid. The expression is written as,

pH=pKa+log[conjugatebase][acid] (1)

This equation shows that pH of buffer solution is controlled by two major factors. First, Strength of the acid can be expressed on terms of pKa and second, the relative concentration of acid and its conjugate base at equilibrium. It can be seen from equation (1) that pH of the buffer solution is comparable to pKa values. So this equation can be used to establish a relation between pH and pKa value of acid.

Explanation

The equilibrium between H3PO4 and its conjugate base NaH2PO4 is written as,

H3PO4(aq)+H2O(l)H3O+(aq)+NaH2PO4(aq)  (acid)                                              (conjugate base)

The pair of H3PO4andNaH2PO4 has the pKa value 2.12 that is very less to as compared to pH value 7

(b)

Interpretation Introduction

Interpretation:

A buffer solution is to be prepared with pH value approximately 7.0. The following pair is suitable, NaH2PO4andNa2HPO4 has to be predicted.

Concept introduction:

The Henderson-Hasselbalch equation relates pH of a buffer with pKa of acid, concentration of conjugate base and concentration of acid. The expression is written as,

pH=pKa+log[conjugatebase][acid] (1)

This equation shows that pH of buffer solution is controlled by two major factors. First, Strength of the acid can be expressed on terms of pKa and second, the relative concentration of acid and its conjugate base at equilibrium. It can be seen from equation (1) that pH of the buffer solution is comparable to pKa values. So this equation can be used to establish a relation between pH and pKa value of acid.

(c)

Interpretation Introduction

Interpretation:

A buffer solution is to be prepared with pH value approximately 7.0. The following pair is suitable,Na2HPO4and Na3PO4 has to be predicted.

Concept introduction:

The Henderson-Hasselbalch equation relates pH of a buffer with pKa of acid, concentration of conjugate base and concentration of acid. The expression is written as,

pH=pKa+log[conjugatebase][acid] (1)

This equation shows that pH of buffer solution is controlled by two major factors. First, Strength of the acid can be expressed on terms of pKa and second, the relative concentration of acid and its conjugate base at equilibrium. It can be seen from equation (1) that pH of the buffer solution is comparable to pKa values. So this equation can be used to establish a relation between pH and pKa value of acid.

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