   Chapter 16, Problem 17PS

Chapter
Section
Textbook Problem

Several acids are listed here with their respective equilibrium constants:C6H5OH(aq) + H2O(ℓ) ⇄ H3O+(aq) + C6H5O−(aq) Ka = 1.3 × 10−10HCO2H(aq) + H2O(ℓ) ⇄ H3O+(aq) + HCO2−(aq) Ka = 1.8 × 10−4HC2O4−(aq) H2O(ℓ) ⇄ H3O+(aq) + C2O42−(aq) Ka = 6.4 × 10−5 (a) Which is the strongest acid? Which is the weakest acid? (b) Which acid has the weakest conjugate base? (c) Which acid has the strongest conjugate base?

(a)

Interpretation Introduction

Interpretation: The strongest acid and the weakest acid has to be identified.

Concept introduction:

Strong acids dissociate completely in water which produce H+ ion, while weak acids dissociate partially in water and produce H+ ion.

An equilibrium constant (K) is the ratio of concentration of products and reactants raised to appropriate stoichiometric coefficient at equlibrium.

For the general acid HA,

HA(aq)+H2O(l)H3O+(aq)+A(aq)

The relative strength of an acid and base in water can be also expressed quantitatively with an equilibrium constant as follows:

Ka=[H3O+][A][HA]

An equilibrium constant (K) with subscript a indicate that it is an equilibrium constant of an acid in water.

Explanation

For acetic acid (HCOOH),

HCOOH(aq)+H2O(l)H3O+(aq)+HCOO(aq)

Ka=[H3O+][HCOO][HCOOH]

A large value of Ka=1.8×104 indicates that the ionization product is strongly favoured because the product [H3O+][HCOO] is more than the equilibrium concentration of the strong acid [HCOOH].

For phenol (C6H5OH),

C6H5OH(aq)+H2O(l)

(b)

Interpretation Introduction

Interpretation: The weakest conjucate base has to be identified.

Concept introduction:

Bronsted –Lowry conjugated acid-Base pairs:

When an acid is dissolved in water, the acid (HA) donates a proton to water to form a new acid (conjugated acid) and a new base (conjugated base). The pair of an Acid –Base differs by a proton called conjugated Acid-Base pair.

(c)

Interpretation Introduction

Interpretation: The strongest conjucate base has to be identified.

Concept introduction:

Bronsted –Lowry conjugated acid-Base pairs:

When an acid is dissolved in water, the acid (HA) donates a proton to water to form a new acid (conjugated acid) and a new base (conjugated base). The pair of an Acid –Base differs by a proton called conjugated Acid-Base pair.

An equilibrium constant (K) is the ratio of concentration of products and reactants raised to appropriate stoichiometric coefficient at equlibrium.

For the general acid HA,

HA(aq)+H2O(l)H3O+(aq)+A(aq)

The relative strength of an acid and base in water can be also expressed quantitatively with an equilibrium constant as follows:

Ka=[H3O+][A][HA]

An equilibrium constant (K) with subscript a indicate that it is an equilibrium constant of an acid in water.

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