   Chapter 16, Problem 48PS

Chapter
Section
Textbook Problem

Methylamine, CH3NH2, is a weak base.CH3NH2(aq) + H2O(ℓ) ⇄ CH3NH3+(aq) + OH−(aq)If the pH of a 0.065 M solution of the amine is 11.70, what is the value of Kb?

Interpretation Introduction

Interpretation:

The base ionization constant (Kb) of weak base methylamine has to be ientified.

Concept introduction:

The base dissociation constant or base ionization constant of weak base methylamine. can be calculated by using the pOH value of methylamine. base (which gives the concentration of hydroxide ion) and from the given concentration of. weak base methylamine.

Explanation

Given data

The pH of the methylamine.base is 11.70

The concentration of methylamine base is 0.065M

Calculate the base dissociation constant (Kb) for the base as follows -

It is known that:pH + pOH = 14

pOH = 14 - pH =14 - 11.70=2.3

Therefore, [OH] can be calculated as:

pOH=log[OH]2.30=log[OH][OH]=5.0×103M

The ICE Table for the dissociation of methylamine base is represented as below:

EquilibriumCH3NH2(aq)+H2O(l)CH3NH3+(aq)+OH(aq)Initial(M)0.06500Change(M)x+x+xEquilibrium(M)0

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