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Question

Calculate the degree of ionization of 0.05M acetic acid if its pKa value is 4.74. 
How is the degree of dissociation affected when its solution also contains
0.01 M

0.1 M in HCl?


Solution

c=0.05MpKa=4.74pKa=log(Ka)Ka=1.82×105Ka=cα2  α=Kacα=1.82×1055×102=1.908×102
When HCl is added to the solution, the concentration of H+ ions will increase. Therefore, the equilibrium will shift in the backward direction i.e., dissociation of acetic acid will decrease. 

When 0.01 M HCl is taken.
Let x be the amount of acetic acid dissociated after the addition of HCl.
CH3COOHH++CH3COOInitial conc.0.05M00After disociation0.05x0.01+xx
As the dissociation of a very small amount of acetic acid will take place, the values i.e., 0.05 - x and 0.01 + x can be taken as 0.05 and 0.01 respectively. 
ka=[CH3COO2][H+][CH3COOH]Ka=(0.01)x0.05x=1.82×105×0.050.01x=1.82×103×0.05M
Now,
α=Amount of acid dissociaedAmount of acid taken=1.82×103×0.050.05=1.82×103

c=0.05MpKa=4.74pKa=log(Ka)Ka=1.82×105Ka=cα2  α=Kacα=1.82×1055×102=1.908×102
When HCl is added to the solution, the concentration of H+ ions will increase. Therefore, the equilibrium will shift in the backward direction i.e., dissociation of acetic acid will decrease. 

When 0.1 M HCl is taken. Let the amount of acetic acid dissociated in this case be X. As we have done in the first case, the concentrations of various species involved in the reaction are:

[CH3COOH]=0.05X;0.05M[CH3COO]=X[H+]=0.1+X;0.1MKa=[CH3COO2][H+][CH3COOH] Ka=(0.1)X0.05x=1.82×105×0.050.1x=1.82×104×0.05M
Now,
α=Amount of acid dissociatedAmount of acid taken=1.82×104×0.050.05=1.82×104


Chemistry
NCERT Textbook
Standard XI

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