   Chapter 16, Problem 101GQ

Chapter
Section
Textbook Problem

For each of the following salts, predict whether a 0.10 M solution has a pH less than, equal to, or greater than 7. (a) NaHSO4 (b) NH4Br (c) KClO4 (d) Na2CO3 (e) (NH4)2S (f) NaNO3 (g) Na2HPO4 (h) LiBr (i) FeCl3 Which solution has the highest pH? The lowest pH?

Interpretation Introduction

Interpretation:

To predict whether 0.10M solution has a pH is less than, equal to or greater than 7.

a) NaHSO4          f) NaNO3b) NH4Br             g) Na2HPO4c) KClO4              h) LiBrd) Na2CO3            i) FeCl3e) (NH4)2S

Concept introduction:

• Salts are solid particles that are composed of cations and anions. When these are dissolved in water may or may not alter the pH of a solution due to the cation, anion or both.
• Salts of cations from strong bases and anions of strong acids have no effect of the pH of an aqueous solution.

The solution pH is equal to the 7.

Example:

NaCl(aq), KCl(aq), NaI(aq), NaNO3(aq)

• The salt of a weak base (Cation) and strong acid (anion) dissolves in water to form acidic solutions.

The solution has a pH less than 7.

• The slat of strong base (cation) and weak acid (anion) dissolves in water to form basic solutions.

The solution has a pH is greater than 7.

Explanation

a)

NaHSO4

It is dissociates into Na+ and HSO4- ions into the solution. Sodium ion is the cation of a strong base. Therefore, NaHSO4 is acidic. Therefore, pH of solution is less than 7.

pH < 7.

HSO4-(aq) + H2O(l) SO42-(aq) + H3O+(aq)

Ka=1.2×10-2

b)

NH4Br

It is dissociates into NH4+ and Br- into the solution. Br- is the anion of a strong acid and forms a neutral solution.

NH4+ ion accepts proton from water molecule producing H3O+ in the solution.

Therefore, NH4Br gives a weakly acidic aqueous solution .Hence, pH < 7

NH4+(aq) + H2O(l)NH3(aq) + H3O+(aq)

Ka= 5.6×10-10

c)

KClO4

It is salt of a strong base and strong acid. Therefore, it gives a neutral solution. pH = 7

d)

Na2CO3

It is a slat of the weak acid and strong base. It dissociates into Na+ and CO32- ions in solution. Na+ ions does not affect the solution pH. CO32- the conjugate base of HCO3-. It abstracts a protons from water molecule producing hydroxide ions into the solution. Therefore, solution becomes basic pH > 7.

CO32-(aq) + H2O(l)HCO3-(aq) + OH-(aq)

Kb= 2.1×10-4

e)

(NH4)2S

It is dissociates into NH4+ and S2- ions into the solution.

NH4+(aq) + H2O(l) NH3(aq) + H3O+(aq) Ka=5

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