Information for possible unknown weak acids.
Acid pKa Molar Mass (g/mol)
Sodium hydrogen sulfate 1.99 120.1
Fluoroacetic acid 2.60 78.04
Chloroacetic acid 2.87 94.49
Mandelic acid 3.41 152.1
Acetylsalicylic acid 3.48 180.2
Lactic acid 3.86 90.08
Ascorbic acid 4.17 176.1
Benzoic acid 4.20 122.1
cis-Crotonic acid 4.41 86.09
Succinamic acid 4.54 117.1
Potassium hydrogen phthalate 5.40 204.2
Disodium hydrogen citrate 6.39 263.1
Sodium hydrogen sulfite 7.20 104.1
A student did the potentiometric titration (with a phenolphthalein indicator present). She saw a color change at 8.25 mL of added 0.1000 NaOH. She had started with 0.1000 g of her unknown acid. Five of her volume/ pH measurements are given below:
vol NaOH added (mL) observed pH
2.00 3.71
4.12 4.20
8.25 10.18
10.00 11.71
12.11 12.02
(a) What is the apparent pKa of her acid?
A. 3.71
B. 4.20
C. 10.18
D. 11.71
E. 12.02