She took a representative 250 mL sample of the water for analysis by the Winkler method. An environmental chemist is concerned about dissolved oxygen levels in a nearby pond. To the sample she added 1.00 mL concentrated NaI/NaOH solution, and 1.00 mL of a Mn2+ solution. When the reaction was complete, 2.00 mL of concentration H2SO4 were added. A 20.0 mL aliquot needed 2.88 mL of a 0.0103 M thiosulfate solution to reach the end point. Describe the ppm concentration of O2 in the original sample?