Question

Use the data from the most concentrated solution of acetic acid to calculate the Ka value for acetic acid. Clearly show all of your work below. mot concentrated solution of acetic acid is 0.116m..ph = 2.85

Answer 1

Let a be the dissociation of the weak acetic acid
                            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 c = concentration = 0.1 M

       pH = 2.85

- log[H⁺] = 2.85

      [H⁺] = 10^-2.85

             = 1.41x10^-3 M

But [H⁺] = ca

1.41x10^-3 = 0.116 x a

           a = 0.0122

K_a = ca² = 0.116x(0.0122)²

             = 1.72x10^-5

Therefore K_a = 1.72x10^-5