Untitled - Kelly Walsh High School

Buffers and Other Equilibria 237
weak acid, then the ratio of base to acid can be calculated. The more concentrated
the acid and base components are, the more acid or base can be neutralized
and the less the change in buffer pH. This is a measure of the buffer capacity,
the ability to resist a change in pH.
What is the pH of a solution containing 2.00 mol of ammonia and 3.00 mol of
ammonium chloride in a volume of 1.00 L? K b 1.81 10 5
NH 3(aq) H 2O(l) K NH 4 (aq) OH (aq)
There are two ways to solve this problem. Assume x [OH ] [NH 4 ] that
comes from the reaction of the ammonia. Ammonia is the conjugate base (A )
of the ammonium ion (HA).
K b [NH 4 ] [OH ]
[NH 3 ]
Assume x is small: 1.81 10 5
Here is an alternate solution:
x 1.21 105 and pOH 4.918
pH 14.000 4.918 9.082
pOH pKb log
[3.00] [x]
[2.00]
[NH 4 ]
[NH 3 ]
pKb log(Kb) log(1.81 105 ) 4.742
[3.00]
4.742 log
[2.00]
pOH = 4.918 pH = 9.082
16-3 Titrations and Indicators
[3.00 x] [x]
[2.00 x]
1.81 105
An acid-base titration is a laboratory procedure that we use to determine the
concentration of an unknown solution. We add a base solution of known concentration
to an acid solution of unknown concentration (or vice versa) until an
acid-base indicator visually signals that the endpoint of the titration has been
reached. The equivalence point is the point at which we have added a stoichiometric
amount of the base to the acid.