Chemistry: The Molecular Science
Chemistry: The Molecular Science
5th Edition
ISBN: 9781285199047
Author: John W. Moore, Conrad L. Stanitski
Publisher: Cengage Learning
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Chapter 14, Problem 121QRT
Interpretation Introduction

Interpretation:

The pHof0.00500MNH4[B(OH)4] has to be calculated (a) by using the simplifying assumption regarding x, (b) by solving a quadratic equation and (c) by using the method of successive approximations.

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Answer to Problem 121QRT

The pHof0.00500MNH4[B(OH)4] (a) by using the simplifying assumption regarding x is 10.46, (b) by solving a quadratic equation is 10.45 and (c) by using the method of successive approximations is 10.45.

Explanation of Solution

The solution contains NH4+ and [B(OH)4] ions.  Ka for NH4+ is 5.6×1010 and Kb for [B(OH)4] is 1.7×105.  As Kb>Ka, the solution is basic.

The equation for the equilibrium can be written as given below.

  B(OH)4(aq)+H2O(l)B(OH)3H2O(aq)+OH(aq)

The basic ionization constant for the above equation can be written as given below.

  Kb=[B(OH)3(H2O)][OH][B(OH)3]

A table can be set up as given below.

B(OH)4(aq)B(OH)3H2O(aq)OH(aq)Initialconc.(M)0.0050001.0×107Changeinconc.(M)x+x+xEquilibriumconc.(M)0.00500xxx

The concentration of OH comes from its auto ionization that is 1.0×107.  It can be assumed to be small compared to the change.

By substituting all the values in the above equation, the value of x can be calculated.

  Kb=(x)(x)(0.00500x)=1.7×105

(a) Assuming x is very small, it can be written as 0.00500x0.00500.

Then,

  x2=(1.7×105)(0.00500)x=2.9×104M=[OH]

The pH of the solution can be calculated as follows,

  pOH=log[OH]=log(2.9×104)=3.54.pH=14.00pOH=14.003.54=10.46.

Therefore, the pHof0.00500MNH4[B(OH)4] is 10.46.

(b) The above equation can be rearranged in the form of a quadratic equation as shown below.

  x2=(1.7×105)(0.00500x)x2+(1.7×105)x8.5×108=0x=2.8×104=[OH]

The pH of the solution can be calculated as follows,

  pOH=log[OH]=log(2.8×104)=3.55.pH=14.00pOH=14.003.55=10.45.

Therefore, the pHof0.00500MNH4[B(OH)4] is 10.45.

(c) using the method of successive approximations, it can be written as given below.

  0.00500x=0.005002.9×104M=0.00471M.

Now,

  x2=(1.7×105)(0.00471)x=2.8×104M=[OH]

The pH of the solution can be calculated as follows,

  pOH=log[OH]=log(2.8×104)=3.55.pH=14.00pOH=14.003.55=10.45.

Therefore, the pHof0.00500MNH4[B(OH)4] is 10.45.

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Chapter 14 Solutions

Chemistry: The Molecular Science

Ch. 14.4 - Calculate the pH of a 0.040-M NaOH solution. 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