   Chapter 17, Problem 2PS

Chapter
Section
Textbook Problem

Does the pH of the solution increase, decrease, or stay the same when you (a) add solid sodium oxalate, Na2C2O4, to 50.0 mL of 0.015 M oxalic acid, H2C2O4? (b) add solid ammonium chloride to 75 mL of 0.016 M HCl? (c) add 20.0 g of NaCl to 1.0 L of 0.10 M sodium acetate, NaCH3CO2?

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 undissociated acid and decrease the concentration of H3O+ in the solution, as a result, pH of the solution will increase.

Explanation

An aqueous solution oxalic acid is a weak base  and dissociates as follows,

H2C2O4(aq)2H+(aq)+C2O42(aq)

If solid sodium oxalate Na2C2O4 is added to the solution then the concentration of H+ ions will decrease as Na2C2O4 is a strong electrolyte and will dissociate in aqueous solution giving Na+ and C2O42 ions

(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 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 undissociated acid and decrease the concentration of H3O+ in the solution, as a result, pH of the solution will increase.

(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 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 undissociated acid and decrease the concentration of H3O+ in the solution, as a result, pH of the solution will increase.

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