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

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1 Answer
anor277
Sep 6, 2017

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?

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