Question

How do you calculate Ka?

Answer 1

In fact you measure `K_a`........

Explanation:

For the hypothetical reaction..........

`A+BrightleftharpoonsC+D`

There is a `"rate forward,"` `=k_f[A][B]`......

And a `"rate backward,"` `=k_r[C][D]`.......

And importantly the condition of chemical equilibrium does not specify the cessation of chemical change, but equality of forward and reverse rates (says he, channelling a text book)........

And thus `k_f[A][B]=k_r[C][D]`......

And `k_f/k_r=([C][D])/([A][B])=K_"eq"`, the thermodynamic equilibrium constant.

For acid base behaviour in water, we write......

`HA+H_2OrightleftharpoonsH_3O^+ +A^-`

`K_a=([H_3O^+][A^-])/([HA])`, ............

the `[H_2O]` term may be removed, because it is so large it is effectively constant. Apologies if I have missed the point of your question.......but for strong acids,......

`HA=HX (X!=F), H_2SO_4, HClO_4`,

.......the equilibrium lies to the right......And the extent of equilibrium, as always, is established by measurement.