equivalence point was determined. The half-equivalence point is the half-way point of
the equivalence point which is where both the weak acid and the base had equal
concentrations. By observing Henderson-Hasselbalch’s equation, pH = p
K
a
+ log
A
−
¿
¿
¿
¿
, if the acid and base are identical, they will have a 1:1 ratio, resulting in the pH being
equal to the p
K
a
(the log 1 is equal to zero).
(The three titration curves and three second derivative curves from the Part A Data
Analysis can be found at the beginning of the worksheet.)

PART A: Preparation and Standardization of NaOH
(6 pts)
Table 1
: NaOH volumes at the equivalence and half-equivalence points of each
acetic acid titration, obtained from the second derivative curves.
Trial
NaOH volume (mL) at
equivalence point
NaOH volume (mL) at
half-equivalence point
pH at half-
equivalence point
1
21.83
10.92
3.12
2
19.06
9.53
3.56
3
19.74
9.87
3.74
Average
20.21
10.12
3.47
8
A. M. Bohnsack
Spring 2019 (revised Z. Baranová)

Worksheet & Grading Rubric – Weak Acids and Buffers
2.
(4 pts)
Calculate the
K
a
of your acetic acid solution. Discuss this calculation. Based on the
value of
K
a
, is acetic acid a strong acid or a weak acid? Why?

3.
(3 pts)
Calculate the number of moles of acetic acid that were present in the original
buffer solution prepared in Step 18
of the procedure. Assume no change in volume upon
addition of sodium acetate trihydrate. Discuss this calculation.

4.
(3 pts)
Calculate the number of moles of acetate that were present in the original buffer
solution prepared in Step 18
of the procedure. The molecular mass of sodium acetate
trihydrate is 136.08 g/mol. Discuss this calculation.