   Chapter 16, Problem 18PS

Chapter
Section
Textbook Problem

Several acids are listed here with their respective equilibrium constants:HF(aq) + H2O(ℓ) ⇄ H3O+(aq) + F−(aq) Ka = 7.2 × 10−4HPO42−(aq) + H2O(ℓ) ⇄ H3O+(aq) + PO43−(aq) Ka = 3.6 × 10−13CH3CO2(H) + H2O(ℓ) ⇄ H3O+(aq) + CH3CO2−(aq) Ka = 1.8 × 10−5 (a) Which is the strongest acid? Which is the weakest acid? (b) What is the conjugate base of the acid I IF? (c) Which acid has the weakest conjugate base? (d) 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 the hydrogen flouride (HF),

HF(aq)+H2O(l)H3O+(aq)+F(aq)

Ka=[H3O+][F][HF]

A large value of Ka=7.2×104 indicates that the ionization product is strongly favoured because the product [H3O+][F] is more than the equilibrium concentration of the strong acid [HF].

For the hydrogen phosphate (HPO42),

HPO42(aq)+H2O(l

(b)

Interpretation Introduction

Interpretation: The conjucate base of HF 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 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.

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.

(d)

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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