Common Ion Effects and Buffers Buffer - Substances in a solution

Document Sample
Common Ion Effects and Buffers Buffer - Substances in a solution Powered By Docstoc
					                       Buffers

Buffer - Substances in a solution that resist changes in pH
             a weak acid and its conjugate salt
             a weak base and its conjugate salt
A buffered solution contains 0.50 M acetic acid and 0.50 M
sodium acetate. Determine the pH of the solution if the Ka of
acetic acid is 1.9 x 10 -5.

                  HC2H3O2           H+     +    C2H3O2 -
Initial (M)         0.50             0            0.50
Change (M)           -x              x              x
 Equil (M)          0.50 -x          x           0.50 + x  .50

 Ka = [ H+] [ C2H3O2-]
        [ HC2H3O2 ]

 1.9 x 10 -5 = [ x ] [ 0.50 ]
                     0.50

 x = 1.9 x 10 –5 = H    +       pH = 4.74
A buffered solution contains 0.50 M acetic acid and 0.50 M
sodium acetate. Determine the pH of the solution if the Ka of
acetic acid is 1.9 x 10 -5.

 Ka = [ H+] [ C2H3O2-]
        [ HC2H3O2 ]

 1.9 x 10 -5 = [ x ] [ 0.50 ]
                     0.50

 x = 1.9 x 10 –5 = H    +       pH = 4.74

Note that the H + concentration depends on the ratio of acid to
conjugate in the solution and the Ka

If the ratio of acid to conjugate is 1 (equimolar concentrations),
then H + concentration equals the Ka
             How Does a Buffer Work
A strong acid or strong base added to water results in a
substantial change in pH since each H + or OH – results in a 1:1
change in [H +] or [OH -].

A buffer solution for example contains a weak acid and its
conjugate base:
         NaC2H3O2  Na + + C2H3O2 -
         HC2H3O2  H + + C2H3O2 -
 The significant acid-base species in the buffer are:
              HC2H3O2 and C2H3O2 -
If an acid is added to the solution:
     H + + C2H3O2 -        HC2H3O2
No significant change in H +
The added H + is sequestered by forming HC2H3O2


If a base is added to the solution:
    OH - + HC2H3O2  H2O + C2H3O2 -
No significant change in OH –
The added OH – is sequestered by forming HOH
The weak acid functions to buffer any hydroxide
added to the solution

The conjugate base functions to buffer any
hydrogen ion added to the solution

An effective buffer should have approximately
equal concentrations of the weak acid and its salt
so that it buffers equally against decreases and
increases in pH

				
DOCUMENT INFO
Categories:
Tags:
Stats:
views:4
posted:3/28/2012
language:
pages:6