Question

Calculate the predicted pH of a .1M Solution of each of the following

CH₂ClCOOH with K_a=1.4x10^-3

NaHSO₄ with K_a = 1.2 x 10^-2

Answer 1

Let a be the dissociation of the weak acid,CH₂ClCOOH
                            HA <---> H ⁺ + A^-

initial conc.            c               0         0

change                -ca            +ca      +ca

Equb. conc.         c(1-a)          ca       ca

Dissociation constant , K_a = ca x ca / ( c(1-a)

                                         = c a² / (1-a)

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

So K_a = ca²

==> a = √ ( K_a / c )

Given K_a = 1.4x10^-3

          c = concentration = 0.1 M

Plug the values we get a = 0.118

[H⁺] = ca = 0.1x0.118 = 0.0118 M

pH = - log[H⁺]

     = - log 0.0118

     = 1.93

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Let a be the dissociation of the weak acid,NaHSO₄
                            HA <---> H ⁺ + A^-

initial conc.            c               0         0

change                -ca            +ca      +ca

Equb. conc.         c(1-a)          ca       ca

Dissociation constant , K_a = ca x ca / ( c(1-a)

                                         = c a² / (1-a)

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

So K_a = ca²

==> a = √ ( K_a / c )

Given K_a = 1.2x10^-2

          c = concentration = 0.1 M

Plug the values we get a = 0.346

[H⁺] = ca = 0.1x0.346 = 0.0346 M

pH = - log[H⁺]

     = - log 0.0346

     = 1.46