   Chapter 17, Problem 1PS

Chapter
Section
Textbook Problem

Does the pH of the solution increase, decrease or stay the same when you (a) add solid ammonium chloride to a dilute aqueous solution of NH3? (b) add solid sodium acetate to a dilute aqueous solution of acetic acid? (c) add solid NaCl to a dilute aqueous solution of NaOH?

a)

Interpretation Introduction

Interpretation:

pH of the solution will increase, decrease or remains same has to be predicted.

Concept introduction:

pH of a solution is basically the measure of the molar concentration of the H+ ion in the solution. More the concentration of H+ ion in the solution, lower will be the pH value and more acidic will be the solution.

The expression for pH is given as,

pH=log[H+]

pOH of a solution is basically the measure of the molar concentration of the OH ion in the solution. More the concentration of OH ion in the solution lower will be the value of pOH and more basic will be the solution.

The expression for pOH is given as,

pOH=log[OH]

pH is calculated using following relation.

pH+pOH=14

Common ion effect is defined as the phenomenon of the suppression of the dissociation of weak acid or base in presence of strong electrolyte having a common ion.  The common ion effect changes pH of the solution.

For example, a weak acid HA undergo partial dissociation in aqueous solution and an equilibrium is established between the dissociated and undissociated acid given as follows,

HA(aq.)+ H2O(l)H3O+(aq.)+ A1(aq.)

Now if in the solution strong electrolyte having a common ion H3O+ or  A1 is added then according to the Le-Chatelier’s principle if the concentration of any of the product will increase then the reaction equilibrium will move to that side of the equilibrium that will decrease the effect of increased concentration of the product. Therefore the reaction will move in a backward direction forming more of un-dissociated acid and decrease the concentration of H3O+ in the solution, as a result,pH of the solution will increase.

Explanation

An aqueous solution of ammonia or ammonium hydroxide NH4OH is a weak base  and dissociates as follows,

NH4OH(aq)NH4+(aq)+OH(aq)

If solid ammonium chloride NH4Cl is added to the solution then the concentration of NH4+ ions will increase as NH4Cl is a strong electrolyte and will dissociate in aqueous solution giving NH4+ and Cl ions.

NH4Cl(s)NH4+(aq)+Cl(aq)

As a result due to the common ion effect, the reaction equilibrium will move towards left and [OH] will decrease. Suppression of dissociation of weak base NH4OH occur and pH of the solution will decrease.

Hence, pH value of the aqueous solution of ammonia will decrease upon addition of solid ammonium chloride.

(b)

Interpretation Introduction

Interpretation:

pH of the solution will increase, decrease or remains same has to be predicted.

Concept introduction:

pH of a solution is basically the measure of the molar concentration of the H+ ion in the solution. More the concentration of H+ ion in the solution, lower will be the pH value and more acidic will be the solution.

The expression for pH is given as,

pH=log[H+]

pOH of a solution is basically the measure of the molar concentration of the OH ion in the solution. More the concentration of OH ion in the solution lower will be the value of pOH and more basic will be the solution.

The expression for pOH is given as,

pOH=log[OH]

pH is calculated using following relation.

pH+pOH=14

Common ion effect is defined as the phenomenon of the suppression of the dissociation of weak acid or base in presence of strong electrolyte having a common ion.  The common ion effect changes pH of the solution.

For example, a weak acid HA undergo partial dissociation in aqueous solution and an equilibrium is established between the dissociated and un-dissociated acid given as follows,

HA(aq.)+ H2O(l)H3O+(aq.)+ A1(aq.)

Now if in the solution strong electrolyte having a common ion H3O+ or  A1 is added then according to the Le-Chatelier’s principle if the concentration of any of the product will increase then the reaction equilibrium will move to that side of the equilibrium that will decrease the effect of increased concentration of the product. Therefore the reaction will move in a backward direction forming more of un-dissociated acid and decrease the concentration of H3O+ in the solution, as a result, pH of the solution will increase.

Explanation

Acetic acid CH3COOH is a weak acid and it dissociates as follows in water,

CH3COOH(aq)CH3COO(aq)+H+(aq)

If solid sodium acetate NaCH3CO2 is added to the solution then the concentration of CH3COO ions will increase as NaCH3CO2 is a strong electrolyte and will dissociate in aqueous solution giving CH3COO and Na+ ions.

NaCH3CO2(s)Na+(aq)+CH3CO2(aq)

As a result due to the common ion effect, the reaction equilibrium will move towards left and [H+] will decrease. Suppression of dissociation of weak acid CH3COOH occur and pH of the solution will increase.

Hence, pH value of the aqueous solution of acetic acid will increase upon addition of solid sodium acetate.

c)

Interpretation Introduction

Interpretation:

pH of the solution will increase, decrease or remains same has to be predicted.

Concept introduction:

pH of a solution is basically the measure of the molar concentration of the H+ ion in the solution. More the concentration of H+ ion in the solution, lower will be the pH value and more acidic will be the solution.

The expression for pH is given as,

pH=log[H+]

pOH of a solution is basically the measure of the molar concentration of the OH ion in the solution. More the concentration of OH ion in the solution lower will be the value of pOH and more basic will be the solution.

The expression for pOH is given as,

pOH=log[OH]

pH is calculated using following relation.

pH+pOH=14

Common ion effect is defined as the phenomenon of the suppression of the dissociation of weak acid or base in presence of strong electrolyte having a common ion.  The common ion effect changes pH of the solution.

For example, a weak acid HA undergo partial dissociation in aqueous solution and an equilibrium is established between the dissociated and un-dissociated acid given as follows,

HA(aq.)+ H2O(l)H3O+(aq.)+ A1(aq.)

Now if in the solution strong electrolyte having a common ion H3O+ or  A1 is added then according to the Le-Chatelier’s principle if the concentration of any of the product will increase then the reaction equilibrium will move to that side of the equilibrium that will decrease the effect of increased concentration of the product. Therefore the reaction will move in a backward direction forming more of un-dissociated acid and decrease the concentration of H3O+ in the solution, as a result, pH of the solution will increase.

Explanation

Sodium hydroxide NaOH is a strong base and it dissociates completely into its constituent ions as follows in water,

NaOH(aq)Na+(aq)+OH(aq)

Addition of solid sodium chloride NaCl to the solution does not affect pH because the common ion effect only suppresses the dissociation of weak acid or base in presence of strong electrolyte. Common ion effect does not affect the dissociation of strong acid or base.

The pH value of the aqueous solution of ammonium hydroxide will decrease upon addition of solid sodium chloride.

Therefore, the pH value of the aqueous solution of ammonium hydroxide will remain same upon addition of solid sodium chloride. pH of the solution does not change.

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