   Chapter 17, Problem 15PS

Chapter
Section
Textbook Problem

A buffer is composed of formic acid and its conjugate base, the formate ion. (a) What is the pH of a solution that has a formic acid concentration of 0.050 M and a sodium formate concentration of 0.035 M? (b) What must the ratio of acid to conjugate base be to have a pH value 0.50 units higher than the value calculated in part (a)?

a)

Interpretation Introduction

Interpretation:

For the given buffer solution of formic acid/ formate ion the value of pH when 0.05MHCOOH and 0.035MHCOO are mixed has to be calculated.

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.

Explanation

The calculation of pH is done by by using Henderson-Hasselbalch equation. The equilibrium between formic acid and its conjugate base formate is written as,

HCOOH(aq)+H2O(l)H3O+(aq)+HCOO(aq)  (acid)                                               (conjugate base)

Given:

Refer to table 16.2 in the textbook for the value of Ka.

The value of Ka for HCOOH is 1.8×104.

Negative logarithm of the Ka value gives the pKa value of the acid.

pKa=log(Ka)=log(1.8×104)=3.74

Therefore, pKa value for the HCOOH is 3

(b)

Interpretation Introduction

Interpretation:

For the given buffer solution of formic acid/ formate ion, the concentration ratio for the formic acid and formate ion when pH value of the buffer is 0.50 units more than the value of pH in part (a) 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.

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