
Concept explainers
(a)
Interpretation:
pH of the solution has to be calculated for 0.65 M potassium formate (HCOOK).
Concept introduction:
An equilibrium constant (K) is the ratio of concentration of products and reactants raised to appropriate
For the general base B,
B(aq)+H2O(l)⇌ BH+(aq)+OH-(aq)
The relative strength of an acid and base in water can be also expressed quantitatively with an equilibrium constant as follows:
Kb=[BH+][OH-][B]
An equilibrium constant (K) with subscript b indicate that it is an equilibrium constant of an base in water.
Base - dissociation constants can be expressed as pKb values,pKb = -log Kb and10 - pKb = Kb
Percent dissociation can be calculated by using following formula,
Percent dissociated = dissociationinitial×100
The Kb value is calculating by using following formula,
Kw = Ka × Kb
(a)

Explanation of Solution
Given,
Potassium formate ion is a base therefore it can accept the proton from the water. Potassium formate dissociates into potassium ions (K+) and formate ions (HCOO–). potassium ions (K+) is the conjugate acid of KOH, so K+ does not react with water. formate ions (HCOO–) is the conjugate base of HCOOH, so it does react with a base-dissociation reaction.
The balance equation is given below,
HCOO-(aq) + H2O(l) ⇌ HCOOH(aq) + OH-(aq)
0.100 M sodium phenolate Solution.
Therefore,
ICE table:
HCOO-(aq) + H2O(l) ⇌ HCOOH(aq) + OH-(aq)
Initial concentration | 0.100 M | - | 0 | 0 |
Change | -x | + x | + x | |
At equilibrium |
0. 0-x | x | x |
The initial concentration is 0.65 M potassium formate (HCOOK).
HCOO-(aq) + H2O(l) ⇌ HCOOH(aq) + OH-(aq) The value Kb of HCOO− is calculating by using following formula, Kw = Ka × KbKb = Kw KaKb = 1×10−14 1.8×10−4Kb = 5.55×10−11Kb =[HCOOH][OH−][HCOO-]given,Kb =[HCOOH][OH−][HCOO-]= 5.55×10−11,let consider,0.65−x= 0.65[HCOOH][OH−][HCOO-]= 5.55×10−11,x2(0.65)= 5.55×10−11x2 = 3.6×10−11x = 6.00×10−6 M = [OH−]
x is small when compared to 0.65 M, therefore, Percent dissociated = dissociationinitial×1006.00×10−6 M 0.65 M×100= 9.24×10−4The dissociation value is very less, and it is validKw=[OH-][H3O+][H3O+]=1.0×10-146.00×10−6 [H3O+]=1.66×10-9Therefore,PH = - log (H3O+)PH = - log (1.66×10-9)PH = 8.77
pH of 0.65 M potassium formate (HCOOK) is 8.77.
(b)
Interpretation:
pH of the solution has to be calculated for 0.85 M NH4Br.
Concept introduction:
An equilibrium constant (K) is the ratio of concentration of products and reactants raised to appropriate stoichiometric coefficient at equlibrium.
For the general base B,
B(aq)+H2O(l)⇌ BH+(aq)+OH-(aq)
The relative strength of an acid and base in water can be also expressed quantitatively with an equilibrium constant as follows:
Kb=[BH+][OH-][B]
An equilibrium constant (K) with subscript b indicate that it is an equilibrium constant of an base in water.
Base - dissociation constants can be expressed as pKb values,pKb = -log Kb and10 - pKb = Kb
Percent dissociation can be calculated by using following formula,
Percent dissociated = dissociationinitial×100
The Kb value is calculating by using following formula,
Kw = Ka × Kb
(b)

Explanation of Solution
Given,
Ammonium bromide dissociates into ammonium ion (NH+4) and bromide ions (Br–). Bromide ion (Br–) is the conjugate base of a strong acid so it will not influence the pH of the solution. Ammonium ion is the conjugate acid of a weak base, so an acid-dissociation reaction determines the pH of the solution.
The balance equation is given below,
NH+4(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq)
0.85 M NH4Br Solution.
Therefore,
ICE table:
NH+4(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq)
Initial concentration | 0.85 M | - | 0 | 0 |
Change | -x | + x | + x | |
At equilibrium | 0.85-x | x | x |
The initial concentration is 0.15 M methylammonium bromide (CH3NH3Br).
NH+4(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq) The value Ka of NH+4 is calculating by using following formula, Kw = Ka × KbKa = Kw KbKa = 1×10−14 1.76×10−5Ka = 5.68×10−10Ka =[ CH3NH2][H3O+][NH+4]given,Ka =[ CH3NH2][H3O+][NH+4]= 5.68×10−10,let consider,0.850−x= 0.850[ CH3NH2][H3O+][NH+4]= 5.68×10−10,x2(0.850)= 5.68×10−10x2 = 4.82×10−10x = 2.19×10−5 M
x is small when compared to 0.850 M, therefore,Percent dissociated = dissociationinitial×100 2.19×10−5 M 0.850 M×100= 2.58×10−3The dissociation value is very less, and it is validTherefore,PH = - log (H3O+)PH = - log (2.19×10−5)PH =4.66
pH of 0.85 M NH4Br is 4.66.
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Chapter 18 Solutions
Student Solutions Manual For Silberberg Chemistry: The Molecular Nature Of Matter And Change With Advanced Topics
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