Vitamin C (L-ascorbic acid) is a six-carbon, 5-membered furan ring with a 1,2-enediol group. It is characterized by two pKa values 4.2 and 11.6, but is only weakly acidic. A student interested in determining the amount of vitamin C in a vitamin tablet dissolves a 800 mg tablet in 50 mL of water. She titrates with a 1*10-2 M solution of NaOH and finds that 35 mL of NaOH reaches the first equivalence point. What is the pH of the NaOH solution before the titration began?
A. 2
B. 5
C. 9
D. 12
Click for Explanation
Two relationships are necessary for this solution:
pOH = -log[OH–] and pOH + pH = 14
pOH = -log[1×10-2] = 2, and 14 – 2 = 12.
A. 2, incorrect, This is the pOH.
B. 5, incorrect, This is the result of subtracting 2 from 7.
C. 9, incorrect, This results from adding the pOH to 7.
D. correct
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