You are a quality control inspector for a nutritional products manufacturer. In
ID: 508171 • Letter: Y
Question
You are a quality control inspector for a nutritional products manufacturer. In the course of your job, you randomly select one sample of Vitamin C tablets about to be shipped to your distributor. The bottle indicates that each tablet contains 500. mg of Vitamin C, also known as ascorbic acid. (H2C6H6O6, Ka1=7.9 x 10-5, Ka2 = 1.6x10-12) Your laboratory technician dissolves one tablet in approximately 6 oz of hot water, to a final volume of 200. mL and titrates this solution with 0.100 M calcium hydroxide. She reports to you the volumes of base added to the beaker were as follows:
mL base added: pH:
0.0
3.3
7.1
10.6
14.2
17.8
21.3
25.2
28.4
34.7
For each of those points in the titration, what pH would you expect to be measured, if the Vitamin C tablets are indeed correctly labeled?
Explanation / Answer
molarity of Vit C solution = 0.5 g/176.124 g/mol x 0.2 L
= 0.0142 M
Titration with 0.1 M Ca(OH)2 (dibasic)
Base (ml) = 0 ml
H2C6H6O6 + H2O <==> HC6H6O6- + H3O+
x moles of acid dissociated
Ka1 = [HC6H6O6-][H3O+]/[H2C6H6O6]
7.9 x 10^-5 = x^2/0.0142
x = [H3O+] = 1.06 x 10^-3 M
pH = -log[H3O+] = 2.975
Base (ml) = 3.3 ml
moles acid = 0.0142 M x 0.2 L = 0.00284 mmol
acid needs = 2 x 0.00284 = 0.00568 mmol
moles base = 0.1 M x 3.3 ml = 0.33 mmol
excess Ca(OH)2 = 0.32432 mmol/0.2033 = 1.6 M
[OH-] = 2 x 1.6 = 3.2 M
pOH = -log[OH-] = 0.50
pH = 14 - pOH = 13.50
Similarly we can calculate for other volumes of base added
Base (ml) pH
7.1 13.85
10.6 13.67
14.2 13.55
17.8 13.45
21.3 13.37
25.2 13.30
28.4 13.24
34.7 13.16
Ca(OH)2 is excess in all the cases.
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