Calculate the pH of a 0.200- M solution of C 5 H 5 NHF. Hint: C 5 H 5 NHF is a salt composed of C 5 H 5 NH + and F − ions. The principal equilibrium in this solution is the best acid reacting with the best base; the reaction for the principal equilibrium is C 5 H 5 NH + ( a q ) + F − ( a q ) ⇌ C 5 H 5 N ( a q ) + HF ( a q ) K = 8.2 × 10 − 3

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Interpretation Introduction Interpretation: The concentration of C 5 H 5 NHF is given. The pH value is to be calculated. Concept introduction: The measure of concentration of hydrogen ions is known as pH . Explanation Explanation To determine : The pH value of C 5 H 5 NHF . The value of K a for C 5 H 5 NHF is 5 . 8 8 × 1 0 − 6 _ . Given The concentration of C 5 H 5 NHF is 0 .200 M . The C 5 H 5 NHF is a salt that dissociates into C 5 H 5 NH + and F − . The ion C 5 H 5 NH + is a conjugate acid of base C 5 H 5 NH 2 whereas F − is the conjugate base of acid HF . The base dissociation constant of C 5 H 5 NH 2 is 1 .7 × 10 − 9 . The relation between K a and K b is, K w = K a × K b (1) Where, K a is acid dissociation constant. K b is base dissociation constant. K w is ion product constant of water ( 1 .0 × 10 − 14 ) Substitute the values of K b and K w in the above equation. K w = K a × K b K a = 10 − 14 1 .7 × 10 − 9 = 5 . 8 8 × 1 0 − 6 _ The value of K b for HF is 1 . 3 8 × 1 0 − 1 1 _ . The acid dissociation constant of HF is 7 .2 × 10 − 4 . Substitute the values of K a and K w in the equation (1). K w = K a × K b K b = 10 − 14 7 .2 × 10 − 4 = 1 . 3 8 × 1 0 − 1 1 _ The concentration of [ H 3 O + ] is 1 . 0 9 × 1 0 − 3 M _ . Since, K a is greater than K b , the solution will be acidic. C 5 H 5 NH + ( a q ) + H 2 O ( l ) ⇌ C 5 H 5 N ( a q ) + H 3 O + ( a q ) The initial concentration of C 5 H 5 NHF is 0

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Chemistry: An Atoms First Approach

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Chemistry: An Atoms First Approach

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Steven S. Zumdahl + 1 other

Publisher: Cengage Learning

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Interpretation Introduction

Interpretation: The concentration of C5H5NHF is given. The pH value is to be calculated.

Concept introduction: The measure of concentration of hydrogen ions is known as pH .

Explanation of Solution

Explanation

To determine: The pH value of C5H5NHF .

The value of Ka for C5H5NHF is 5.88×10−6_ .

Given

The concentration of C5H5NHF is 0.200 M .

The C5H5NHF is a salt that dissociates into C5H5NH+ and F− . The ion C5H5NH+ is a conjugate acid of base C5H5NH2 whereas F− is the conjugate base of acid HF .

The base dissociation constant of C5H5NH2 is 1.7×10−9 .

The relation between Ka and Kb is,

Kw=Ka×Kb (1)

Where,

Ka is acid dissociation constant.
Kb is base dissociation constant.
Kw is ion product constant of water (1.0×10−14)

Substitute the values of Kb and Kw in the above equation.

Kw=Ka×KbKa=10−141.7×10−9=5.88×10−6_

The value of Kb for HF is 1.38×10−11_ .

The acid dissociation constant of HF is 7.2×10−4 .

Substitute the values of Ka and Kw in the equation (1).

Kw=Ka×KbKb=10−147.2×10−4=1.38×10−11_

The concentration of [H3O+] is 1.09×10−3 M_ .

Since, Ka is greater than Kb , the solution will be acidic.

C5H5NH+(aq)+H2O(l)⇌C5H5N(aq)+H3O+(aq)

The initial concentration of C5H5NHF is 0

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