1.1 What is Chemistry?

21.5. Salt Solutions http://www.ck12.org

If we dissolved both acetic acid and some source of acetate anion (such as sodium acetate) in water, the resulting
solution would be set up to absorb small amounts of added acid or base without major changes to the concentration
of H+. For example, if a strong acid was added, most of the resulting H+ions would combine with acetate ions to
make molecules of acetic acid. The addition of a strong base would cause mostly deprotonation of the acetic acid,
and very little increase in the concentration of the hydroxide ion.

If we know the concentration of both the acid and its conjugate base, we can calculate the pH of the buffer based
on the ratio between these two components. For example, say we made a solution that contained 0.10 M acetic acid
and 0.20 M sodium acetate. The equilibrium expression for this reaction has the following form:

Ka=[CH^3 COO

−][H+]

[CH 3 COOH]

Plugging in the known concentrations, along with the Kaof acetic acid (1.74× 10 −^5 ) allows us to solve for the
concentration of H+:

1. 74 × 10 −^5 =[^0.^20 ][H

+]

[ 0. 10 ]

[H+] =[[^00 .. 2010 ]]( 1. 74 × 10 −^5 )

[H+] = 8. 70 × 10 −^6 M

Then, we can calculate the pH as usual:

pH=−log[H+]

pH=−log[ 8. 70 × 10 −^6 ]

pH= 5. 06

This buffer would have a pH of 5.06. We can tailor the buffer to a specific pH by changing the ratio of acid to salt in
the mixture.

Note that if we have equal concentrations of the acid and its conjugate base, the equilibrium constant expression can
be simplified as follows:

Ka=[CH^3 COO

−][H+]

[CH 3 COOH]

Ka=[x][H

+]

[x]

Ka= [H+]

This also means that the pH can be determined directly from the Ka:

pH=−log[H+]

pH=−log(Ka)

The negative logarithm of the Kais known as thepKaof an acid. Tables of acid strengths often list pKavalues
instead of Kavalues so that exponents are not required. As with pH, a lower pKaindicates a more acidic compound.

Lesson Summary

• A salt formed from a weak acid and a strong base will form a basic solution (pH >7), because the anion acts
  as a weak base.


• A salt formed from a strong acid and a weak base will form an acidic solution (pH <7), because the cation acts
  as a weak acid.