   Chapter 16, Problem 16.68QP General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
ISBN: 9781305580343

Solutions

Chapter
Section General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
ISBN: 9781305580343
Textbook Problem

What is the pH of a 0.30 M solution of methylammonium chloride, CH3NH3Cl? What is the concentration of methylamine in the solution?

Interpretation Introduction

Interpretation:

The concentration of methylamine and pH of a 0.30 M methylammonium chloride solution has to be calculated

Concept Introduction:

Relationship between Ka and Kb

Ka×Kb=Kw

Where, Kw is the auto-ionization of water. (1.0×1014)

Autoionization of water is the reaction in which the water undergoes ionization to give a proton and a hydroxide ion.  The ionization of water is an equilibrium reaction and hence this has equilibrium rate constant.

Kw=[H3O+][OH]=1.0×1014

Relationship between pH and pOH:

pH + pOH = 14

Explanation

To calculate: The concentration of methylamine and pH of a 0.30 M methylammonium chloride solution

Given data:

The concentration of methylammonium chloride solution is 0.30 M

Calculation of Ka for methylammonium cation:

The Kb of methylamine is 4.4×104

The Ka can be calculated from the Kb of its conjugate base as follows,

Ka =KwKb =1.0×10-144.4×104 =2.27×1011

Therefore, the Ka of methylammonium ion is 2.27×1011

Calculation of concentration:

Set up the equilibrium table for the given solution.

Let x be the unknown concentration.

Represent, methylammonium cation as MeNH3+ and methylamine as MeNH2

Methylammonium chloride dissociates as MeNH3+ and Cl- ions. The hydrolysis of methylammonium cation is as follows,

 MeNH3+(aq)+H2O(l)     ⇌        H3O+(aq)  +   MeNH2(aq) Initial (M) 0.30 -x 0.30-x 0

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