   Chapter 17, Problem 12PS

Chapter
Section
Textbook Problem

Calculate the pH of a solution that has an ammonium chloride concentration of 0.050 M and an ammonia concentration of 0.045 M.

Interpretation Introduction

Interpretation:

The value of pH has to be calculated for the buffer solution containing 0.05MNH4Cl and 0.045MNH3 by using the Henderson-Hasselbalch equation.

Concept introduction:

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

pOH=pKb+log[conjugateacid][base] (1)

This equation shows that pOH of buffer solution is controlled by two major factors. First, is strength of the base which can be expressed on terms of pKb and second, the relative concentration of base and its conjugate acid at equilibrium.

Explanation

The equilibrium between NH3 and its conjugate base NH4+ is written as follows;

NH3(aq)+H2O(l)NH4+(aq)+OH(aq)(base)(conjugateacid)

The calculation of pOH is done by using Henderson-Hasselbalch equation.

The value of pH is calculated by using expression, pH + pOH = 14.

Given:

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

The value of Kb for NH3 is 1.8×105.

Negative logarithm of the Kb value gives the pKb value of the acid.

pKb=log(Kb)=log(1.8×105)=4.74

Therefore, pKb value for the NH3 is 4.74.

The concentration of NH3 is 0.050molL1.

The concentration of NH4+ is 0

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