Question

Part A

Which of these weak acid solutions has the greatest percent ionization?

Which of these weak acid solutions has the greatest percent ionization?

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Part B

Which solution has the lowest (most acidic) pH?

0.0600 M HC2H3O2

3.00 M HC2H3O2

0.600 M HC2H3O2 Which solution has the lowest (most acidic) pH?

0.600 M HC2H3O2

0.0600 M HC2H3O2

3.00 M HC2H3O2

Answer 1

Part A :

Let α be the dissociation of the weak acid
                            HA <---> H ⁺ + A^-

initial conc.            c               0         0

change                -cα            +cα      +cα

Equb. conc.         c(1-α)          cα    cα

Dissociation constant , K_a = cα x cα / ( c(1-α)

                                         = c α² / (1-α)

In the case of weak acids α is very small so 1-α is taken as 1

So K_a = cα²

==> α = √ ( K_a / c )

Given K_a = 1.8x10^-5

          c = concentration

(a) c = 0.0600 M ----> α = √ ( K_a / c ) = 0.0173

    % dissociation = α x100 = 1.73 %

(b) c = 3.00 M ----> α = √ ( K_a / c ) = 2.45x10^-3

    % dissociation = α x100 = 0.24 %

(c) c = 0.600 M ----> α = √ ( K_a / c ) = 5.477x10^-3

    % dissociation = α x100 = 0.55 %

Among these threee the value of %α is more for 0.0600 M HC2H3O2 it is the required answer

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Part B :

[H⁺] = cα & pH = - log [H⁺]

Calculate these values & compare the pH values