equilibrium and pH?

Question

For propanoic acid (HC3H5O2, Ka = 1.3 10-5), determine the concentration of all species present, the pH, and the percent dissociation of a 0.114 M solution.
[HC3H5O2]
M
[C3H5O2-]
M
[H+]
M
[OH -]
M
pH

percent dissociation
%

Answer 1

If C is your initial concentration (C=0.114 M) then

.. .. .. .. .. .. .. .. HC3H5O2 <=> C3H5O2- + H+
Intial .. .. .. .. .. .. .. C
Dissociate .. .. .. .. x
Produce .. .. .. .. .. .. .. .. .. .. .. .. .. .. x .. .. .. .. x
At Equilbrium .. .. C-x .. .. .. .. .. .. .. x. .. .. .. .. x

Ka=[C3H5O2-][H+] / [HC3H5O2]= x^2/(C-x)

You find x either by solving the quadratic equation or by assuming that C >> x and then C-x=C so Ka=x^2/C. You calculate x and if it is truly x << C the assumption is valid (otherwise you have to solve the quadratic)

Once you have the value of x,
[H+]=[C3H5O2-]=x
[HC3H5O2]=C-x= 0.114-x
pH=-logx
[OH-]= Kw/[H+]= 10^-14/x, or if you prefer pOH=14-pH and [OH=]=10^-pOH

percent dissociation is (x/C)*100%