Passage II
In aqueous solutions, vitamin C (C6H8O6) reacts with iodine (I2) according to the balanced chemical equation
C6H8O6 + I2 → C6H6O6 + 2HI
The amount of vitamin C in a solution can be determined by performing a titration. The titrant (a solution containing a known concentration of I2) is slowly added to the analyte (a solution containing a particular concentration of vitamin C), and a chemical reaction occurs. This process is monitored by adding an indicator to the analyte. The indicator is a colorless starch solution that changes to a dark color by reacting with I2 when no more vitamin C remains, signaling the end of the titration.
Students performed titrations to determine the amount of vitamin C in 4 different types of pulp-free juice.
Experiment 1
A 0.20 mg/mL vitamin C solution was prepared. A 1.0 mL sample of the solution was placed into a vial, and 1 drop of indicator was added to the vial. Drops of the I2 solution were then added to the vial until the indicator changed to a dark color. The procedure was repeated for solutions of 0.40, 0.60, 0.80, and 1.0 mg/mL of vitamin C (see Figure 1) .
Experiment 2
A 1.0 mL sample of lime juice was placed into a vial, and 1 drop of indicator was added to the vial. Drops of the I2 solution were added to the vial until the lime juice solution changed to a dark color. This procedure was repeated for 3 other juices (see Table 1).
Figure and label adapted from R. Ballentine, "Determination of Ascorbic Acid in Citrus Fruit Juices." @ 1941 by the American Chemical Society.
11. Based on the results of Experiments 1 and 2, the concentration of vitamin C in the lemon juice was closest to which of the following?
Your Answer is
Correct Answer is C
Explanation
Looking at Table 1 first, Lemon juice needs to consume 18 drops of I2 solution; then looking at Figure 1, when the ordinate is 18, the corresponding abscissa is about 0.45 mg/mL.