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I-4
EXPERIMENTAL METHOD
An acetic acid-sodium-acetate buffer system will be prepared by combining equal molar quantities,
resulting in a solution with the maximum buffering capacity. Samples of this solution will be used to
determine the effects of dilution and the addition of a strong acid and a strong base on the buffer
system. The effect of these additions will be monitored by observing any changes in the pH of the
solution.
ADVANCE STUDY ASSIGNMENT
1.
Review the previous information on acid-base chemistry.
2.
Review instructions on the use of pH meters. (pages 25 - 27)
3.
What equation might you use to find the ratio of the concentration of the acid to that of its conjugate
base at various pH values?
Answer: the Henderson-Hasselbalch equation
4.
What volume of the 0.250 M Ca(CH3COO)2 (calcium acetate) solution should you add to 25.00
mL
of 0.500 M CH3COOH to prepare a buffer solution which has a pH of 4.754?
Answer: 25.0 mL.
To solve this problem:
(a)
First, calculate the [H3O
+
] that corresponds to the desired pH of 4.754.
Answer: 1.76 x 10
-5
(b)
Then use the weak acid dissociation relation to calculate the ratio of [CH3COO
-
]/[CH3COOH]
required to obtain the desired pH (K
a
is given on the Chemistry Data Sheet).
(c)
Solve for the volume of acetate solution required. (Note the stoichiometric ratio.)
5.
Use the Henderson-Hasselbalch equation to calculate the pH of a buffer solution
(to the correct number of significant figures) in which the concentration of the benzoate ion is
0.0300 M, and the concentration of the benzoic acid is 0.0100 M.
Answer: 4.670.
6.
Calculate the mass of sodium acetate trihydrate (CH3COONa•3 H2O ) required to make 50.0 mL
of a 0.20 M solution.
Answer: 1.36 g
REAGENTS AND APPARATUS
standard buffer solutions: pH 4.00 and pH 7.00
pH meter with electrode
0.20 M HCl (hydrochloric acid)
buret (2 ).
0.2xxx M NaOH (standardized sodium hydroxide)
0.20 M CH3COOH (acetic acid)
0.20 sodium acetate trihydrate (CH3COONa•3 H2O )
DI water with a trace of NaCl
