Passage V

Multivitamin (MV) tablets often contain iron in the form of Fe2+. In aqueous solution, Fe2+ does not absorb visi­ble light. However, when o-phenanthroline (o-phen) is present, it interacts with Fe2+ to form an orange-colored complex, which absorbs visible light at a wavelength of 508 nanometers (nm).

Students measured the absorbance at 508 nm—the A508—of 6 solutions having known Fe2+ concentrations and then determined the Fe2+ content of 4 brands of MV tablets (Brands A-D).

Experiment 1

Steps 1-5 were performed 6 times:

1. A certain volume of an aqueous stock solution having a concentration of 0.04 mg Fe2+/mL was placed in a 100.0 mL flask.

2. Two mL of hydroquinone solution was added to the flask to stabilize Fe2+.

3. Three mL of o-phen solution was added to the flask.

4. The contents of the flask were diluted with H2O to form a solution of 100.0 mL.

5. The A508 of the solution formed was measured with a colorimeter.

Table 1 shows, for each of the 6 solutions formed (Solutions 1-6), the volume of stock solution used, the Fe2+ concentration, and the A508.

Experiment 2

Each of Brands A-D was analyzed by crushing 1 tablet, dissolving the powder in H2O to form a 100.0 mL solution, placing 1 mL of the solution into a flask, and then performing Steps 2-5. From the results, each brand's iron (Fe2+) content, in mg/tablet, was calculated. Table 2 compares, for each brand, the calculated content with the iron (Fe2+) content stated on the label.

31. In Experiment 1, was the volume of H2O that had to be added to the flask in Step 4 to form Solution 2 greater than or less than the volume of H2O that had to be added to the flask in Step 4 to form Solution 5 ?

Answer and Explanation

Your Answer is

Correct Answer is B

Explanation

Looking at Step 4 of EXperiment 1, after the stock solution is diluted, the final solution volume is 100 mL, and it can be seen from table 1 that the stock solution in solution 2 has a volume of 1 mL, while the stock solution in solution 5 has a volume of 1 mL The volume of the stock solution is 4 mL;

Because in the experimental operation, 2 mL of hydroquinone and 3 mL of o-phen solution were added to each solution, so in the process of diluting the stock solution to form solution 2 , 100-1-(2+3)=94 mL of water was added. In the process of diluting the stock solution to form solution 5, 100-4-(2+3)=91 mL of water was added, so since the stock solution of solution 2 has less content, it is necessary to add more water to prepare 100 mL of solution 2 water