Question

table 1: Equilibrium Constants Data
Syringe Reading	pH After Each 0.5 mL Increment	Color Observations
0	1.7	Clear yellow
2.5	2	yellow
5	2.5	Light yellow
7.5	3	off yellow
10	3.4	off yellow
12.5	3.8	off yellow
15	4	off orange
17.5	4.2	off orange
20	4.5	off orange
22.5	4.8	off orange
25	7.1	0range
27.5	9	Off orange
30	9.2	Off orange
32.5	9.4	Off orange
35	9.5	pink
37.5	9.7	off pink
40	10	Pink
42.5	10.3	Pink
45	10.5	Dark pnik
47.5	11	Dark pink
49	11.5	Na
50	12.1	na

use the graph to answer all questions, we are doing equillibrium constant

(a) According to your experimental data, what volume of 0.10 M NaOH represents the half-equivalence (a.k.a. half-neutralization) point in this titration?

(b) What is the pH of the solution at the half-neutralization point?

(c) Using this info, what is the experimental pK_a for acetic acid in this reaction? Explain your answer.

(d) Using the pK_a value above, what is your experimental K_a for acetic acid? (You must show all work to receive credit)

e Look up the accepted “actual” K_a value for acetic acid. How does your value compare? Calculate the percent error for your experimental value. (You must show all work to receive credit.)

Answer 1

a) volume of NaOH at half-neutralization point = 12.5 ml

b) pH = 3.75 at half-neutralization point

c)according to Henderson equation

pH = pKa + log { [salt]/[acid]} ( because this is a buffer system)

at half neutralization point half of initial concentration of acid is converted to the salt. sao, at this point [salt] - [acid]

therefore, pH = pKa = 3.75

d)pKa = - log Ka

so, Ka = 1.78*10^-4