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The ionization constant for acetic acid is 1.8 x 10-5.
(a) Calculate the concentration of H+ ions in a 0.10 molar solution of acetic acid.
(b) Calculate the concentration of H+ ions in a 0.10 molar solution of acetic acid in which the concentration of acetate ions has been increased to 1.0 molar by addition of sodium acetate.
Solution:
Since acetic acid is a weak acid, then the ionization is not 100% complete. Some molecules of acetic acid will be ionized into acetate and hydrogen ions and the rest of the molecules of acetic acid will remain the same at the end of ionization process. The ionization process of acetic acid is
and its equation for getting the ionization constant is
The concentration of hydrogen ion, acetate ion, and acetic acid are expressed in molarity.
(a) At the start of the ionization process
Let 0 = be the concentration of [H+]
0 = be the concentration of [C2H3O2-]
0.1 = be the concentration of [HC2H3O2]
At the end of the ionization process
Let x = be the concentration of [H+]
x = be the concentration of [C2H3O2-]
0.1 - x = be the concentration of [HC2H3O2]
Therefore, the concentration of [H+] at the end of the ionization process is
Since the value of Ka is very small, then we can neglect x at the denominator so that we don't have to use quadratic formula. If the value of Ka is 1 x 10-3 and above, then we have to solve for the value of x by quadratic formula.
Hence, the value of x which is the concentration of [H+] is
(b) After the addition of sodium acetate to the solution, the concentration of acetate ion has been increased to 1.0 M. Therefore, the concentration of [H+] is
Again, we can neglect x at the denominator since the value of Ka is very small.
Hence, the value of x which is the concentration of [H+] is
Did you noticed that the amount of [H+] becomes smaller after the addition of sodium acetate? Well, if you increase the concentration of either one of the reactants and products, then the concentration of the other substances or components will be affected because of Le Chatelier's Principle.