   Chapter 17, Problem 11PS

Chapter
Section
Textbook Problem

Calculate the pH of a solution that has an acetic acid concentration of 0.050 M and a sodium acetate concentration of 0.075 M.

Interpretation Introduction

Interpretation:

The value of pH is to be calculated for the buffer solution containing 0.05MCH3COOH and 0.075MCH3COONa by using the Henderson-Hasselbalch equation.

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, is strength of the acid which can be expressed on terms of pKa and second, the relative concentration of acid and its conjugate base at equilibrium.

Explanation

The equilibrium between CH3COOH and its conjugate base CH3COONa is written as follows;

CH3COOH(aq)+H2O(l)H3O+(aq)+CH3COO(aq)(acid)(conjugatebase)

The calculation of pH is done by using Henderson-Hasselbalch equation is given below.

Given:

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

The value of Ka for CH3COOH is 1.8×105.

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

pKa=log(Ka)=log(1.8×105)=4

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