Question

Can you calculate the acid dissociation constant (Ka) of a weak acid from experimental data?

Answer 1

Yes, we can, but first we need some experimental data.......and someone must first do the measurement....

Explanation:

We interrogate the equilibrium....

`HA(aq)+H_2O(aq) rightleftharpoonsA^(-) + H_3O^+`

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

Now it is a fact, that `[H_3O^+]` for a `1.0*mol*L^-1` solution of `"acetic acid"` is `4.24xx10^-3*mol*L^-1`. And I got these data from the interwebz; of course someone had to measure these data.....

`H_3C-CO_2H(aq)+H_2O(aq) rightleftharpoonsH_3C-CO_2^(-) + H_3O^+`

And `K_a=([H_3O^+][H_3C-CO_2^-])/([H_3C-CO_2H])`

`=(4.24xx10^-3*mol*L^-1xx4.24xx10^-3*mol*L^-1)/(1.0*mol*L^-1-4.24xx10^-3*mol*L^-1)`

`=1.81xx10^-5`

`pK_a=-log_10(1.81xx10^-5)=??`

Capisce?