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The Ionization Constant of Phenol is 1.0 × 10–10. What is the Concentration of Phenolate Ion in 0.05 M Solution of Phenol? What Will Be Its Degree of Ionization If the Solution is Also 0.01m in Sodium Phenolate? - CBSE (Science) Class 11 - Chemistry

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Question

The ionization constant of phenol is 1.0 × 10–10. What is the concentration of phenolate ion in 0.05 M solution of phenol? What will be its degree of ionization if the solution is also 0.01M in sodium phenolate?

Solution 1

Ionization of phenol:

                              `C_6H_5OH + H_2O  ↔C_6H_5O^(-) + H_3O^(+)`

Initial conc                    0.05                             0               0

At equilibrium           0.05 - x                           x                x

`K_a = ([C_6H_5O^(-)][H_3O^(+)])/[C_6H_5OH]`

`K_a = (x xx x)/(0.05 - x)`

As the value of the ionization constant is very less, x  will be very small. Thus we can ignore x in the denominator

`:. x = sqrt(1xx10^(-10) xx 0.05)`

`= sqrt(5xx10^(-12))`

`= 2.2 xx 10^(-6) M = [H_3O^(+)]`

Since `{H_3O^+] = [C_6H_5O^(-)]`

`[C+6H_5O^(-)] = 2.2 xx 10^(-6) M`

Now, let ∝ be the degree of ionization of phenol in the presence of 0.01 M C6H5ONa.

`C_6H_5ONa -> C_6H_5O^(-) + Na^(+)`

Conc                               0.01

Also

                `C_6H_5OH   + H_2O ↔ C_6H_5O^(-) + H_3O^(+)`

Conc   `0.05-0.05alpha`                    `0.05alpha`     `0.05alpha`

`[C_6H_5OH] = 0.05 - `0.05alpha` ; 0.05 M`

`[C_6H_5O^(-)] = 0.01 + 0.05 alpha 0.01 M`

`H_3O^+] = 0.05 alpha`

`K_a = ([C_6H_5O^(-)][H_3O^(+)])/[C_6H_5OH]`

`K_a = ((0.01)(0.05alpha))/0.05`

`1.0 xx 10^(-10) = .01 alpha`

`alpha = 1 xx 10^(-8)`

Solution 2

                       `C_6H_5OH ⇌ C_6H_5O^(-) + H^(+)`

Initial                  0.05 N

After disso         0.05-x          xx

:. `K_a = (x xx x)/(0.05 - x) = 1.0 xx 10^(-10)` (Given) or `x^2/0.05 = 1.0 xx   10^(-10)`

or `x^2 = 5 xx 10^(-12) or x = 2.2 xx 10^(-6) M`

In presence of 0.01 `C_6H_5ONa` suppose is the amount of phenol dissociate, then at equilibrium

`[C_6H_5OH] = 0.05 - y ~~ 0.05`

`[C_6H_5O^(-)] = 0.01 + y ~~ 0.01 M, [H^+] = yM`

`:. K_a = ((0.01)(y))/0.05 = 1.0 xx 10^(-10)` Given or y = `5 xx 10^(-10)`

`:. alpha = y/c = (5xx10^(-10))/(5xx10^(-2)) = 10^(-8)`

  Is there an error in this question or solution?

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Solution The Ionization Constant of Phenol is 1.0 × 10–10. What is the Concentration of Phenolate Ion in 0.05 M Solution of Phenol? What Will Be Its Degree of Ionization If the Solution is Also 0.01m in Sodium Phenolate? Concept: Ionization of Acids and Bases - Ionization Constants of Weak Acids.
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