Chem 121
Homework 8
Due 12/2/08
1. The following concerns the acid/base equation:
HCOOH (aq) + H2O (l) <----> H3O+ (aq) + HCOO- Ka = 1.8*10-4
(a) Identify the two sets of conjugate acid: base pairs (label which is the acid and
which is the base in each pair).
(b) Is HCOOH a strong acid or a weak acid? What does that imply about its
conjugate base?
(c) Write the Ka expression for HCOOH
(d) Do you expect there to be more HCOO- or HCOOH at equilibrium? Why?
(e) You add some HCOOH to water and test the pH of the solution using pH paper.
Its pH is 6.5. What is the [H3O+] and [OH-] in the solution? Is this solution
acidic, basic or neutral?
(f) Now you make a new solution where you know the [H3O+] = 2.0 * 10-6M. What
is the pH of the solution? Is it acidic, basic or neutral?
2. The following question is adapted from 10.66/10.67 in your book. Consider the buffer
system of nitrous acid, HNO2 and its salt NaNO2.
HNO2(aq) + H2O(l) <----> H3O+ (aq) + NO2a. What is the purpose of a buffer system?
b. What is the purpose of the NaNO2 in the buffer?
c. How does the buffer react when some H3O+ is added
d. How does the buffer react when some OH- is added?
e. Nitrous acid has a Ka of 4.5 * 10-4. Wht is the pH of a buffer solution containing
0.25M HNO2 and 0.25M NO2-?