Passage II

The rate of a chemical reaction can be measured as the change in the concentration of its products over a period of time. The following experiments were performed to investigate factors that influence the rate at which potassium permanganate (KMnO4) reacts with oxalic acid（H2C2O4) to form Mn2+and other products.

Experiment 1

A 4.0 mL sample of 0.1 M (moles/liter) aqueous KMnO4, which was purple in color, was measured with a clean graduated cylinder and poured into a test tube. A 4.0 mL sample of 1.0 M aqueous H2C2O4 was then measured with a clean graduated cylinder and poured into a second test tube. The 2 test tubes were placed in a 25℃ water bath. After 5 minutes the H2C2O4 solution was mixed with the KMnO4 solution and placed back in the water bath. The time until the purple color disappeared was recorded. This procedure was repeated at 3 other temperatures. The results are shown in Table 1.

Experiment 2

The procedure from Experiment 1 was repeated in every way except that 0.1 mg of manganous sulfate（MnSO4), a catalyst, was added to each test tube containing KMnO4 before the H2C2O4 was added. Catalysts are substances that increase the rate of reactions without being used up. The results are shown in Table 2.

Experiment 3

The procedure from Experiment 1 was repeated at a constant temperature of 25°C and various concentrations of H2C2O4. The results are shown in Table 3.

13. A student repeating Trial 9 unknowingly used a graduated cylinder that already contained 1.0 mL of H2O to measure the H2C2O4 solution. Based on the results of Experiment 3, the reaction time that she measured was most likely:

Answer and Explanation

Your Answer is

Correct Answer is D

Explanation

Because there is still 1 mL of water, the Concentration of H2C2O4 must be less than 0.125 in trail 9, so the reaction time must be longer than 837 sec (as can be seen from table 3, the lower the concentration, the longer the reaction time)