How do you calculate Ka?

1 Answer
Jul 17, 2017

Answer:

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.