Hydrogen peroxide decomposes back to water and oxygen when exposed to air and light. When bought at a pharmacy for home use, hydrogen peroxide is sold in dark bottles that are labeled as a concentration of 3% by weight. A chemistry student wants to test the concentration of hydrogen peroxide that has been decanted to light and dark bottles, and exposed to air for 10, 20 and 30 hours. The concentration is tested by titration with potassium permanganate, and each sample is tested twice.