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I-4
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 (mL) 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
buret (2 ).
0.20 M HCl (hydrochloric acid)
0.20 M NaOH (standardized sodium hydroxide)
0.20 M CH3COOH (acetic acid)
sodium acetate trihydrate (CH3COONa•3 H2O )
