Dissolved oxygen levels in natural water

 A water chemist obtained a 250-mL sample from a nearby lake and fixed the oxygen on-site with alkaline solutions of MnSO4 and KI-NaN3. Returning to the laboratory, a 200-mL sample was analyzed by acidifying the sample with conc H2SO4 and then titrating with 14.4 mL of 0.0213 M Na2S2O3 solution to the starch endpoint.

a) Calculate the number of moles of I3 that reacted with the Na2S2O3 solution.

b) Calculate the number of moles of Mn(OH)3 that were produced from the reduction of the dissolved oxygen.

c) Calculate the number of moles and milligrams of O2 present in the titrated sample.

d) What is the dissolved oxygen concentration in the sample, expressed in ppm O2?