Calculate [OH − ] , [H + ], and the pH of 0.20 M solutions of each of the following amines. a. triethylamine [(C 2 H 5 ) 3 N, K b = 4.0 × 10 −4 ] b. hydroxylamine (HONH 2 , K b = 1.1 × 10 −8 )

Interpretation: The pH, [H+] and [OH−] of the given amines is to be calculated.

Concept introduction: The pH of a solution is defined as a figure that expresses the acidity of the alkalinity of a given solution.

The pOH of a solution is calculated by the formula, pOH=−log[OH−]

The sum, pH+pOH=14

At equilibrium, the equilibrium constant expression is expressed by the formula,

Kb=Concentration of productsConcentration of reactants

To determine: The pH, [H+] and [OH−] of (C2H5)3N .

Explanation

The equilibrium constant expression for the given reaction is,

Kb=[(C2H5)3NH+][OH−][(C2H5)3N]

The reaction involved is,

(C2H5)3N(aq)+H2O(l)⇌(C2H5)3NH+(aq)+OH−(aq)

At equilibrium, the equilibrium constant expression is expressed by the formula,

Kb=Concentration of productsConcentration of reactants

Where,

• Kb is the base ionization constant.

The equilibrium constant expression for the given reaction is,

Kb=[(C2H5)3NH+][OH−][(C2H5)3N] (1)

The [OH−] is 8.94×10-3 M_ .

The change in concentration of (C2H5)3N is assumed to be x .

The liquid components do not affect the value of the rate constant.

The ICE table for the stated reaction is,

(C2H5)3N(aq)⇌(C2H5)3NH+(aq)+OH−(aq)Inititial concentration0.2000Change−x+x+xEquilibrium concentration0.20−xxx

The equilibrium concentration of [(C2H5)3N] is (0.20−x) M .

The equilibrium concentration of [(C2H5)3NH+] is x M .

The equilibrium concentration of [OH−] is x M .

The Kb value is given to be 4.0×10−4 .

Substitute the value of Kb , [(C2H5)3N] , [(C2H5)3NH+] and [OH−] in equation (1)

Interpretation: The pH, [H+] and [OH−] of the given amines is to be calculated.

Concept introduction: The pH of a solution is defined as a figure that expresses the acidity of the alkalinity of a given solution.

The pOH of a solution is calculated by the formula, pOH=−log[OH−]

The sum, pH+pOH=14

At equilibrium, the equilibrium constant expression is expressed by the formula,

Kb=Concentration of productsConcentration of reactants

To determine: The pH, [H+] and [OH−] of HONH2 .