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Transcription of this question: The biochemical oxygen demand of a water sample can be determined by the followingseries of reactions. The final step is titration of the sample with sodium thiosulfate solution,Na2S203 (aq).(aq) + 02(aq) + 40K (aq) 2Mn02(s) + 2H20 (l)Mn02(s) + 21- (aq) + 414* (aq) Mn2+ (aq) + 12 (aq) + 2H20 (l)+ 12(aq) 2E(aq) + S40$(aq)A student analysed two 300.0cm3 samples of water taken from the school pond: oneimmediately (day O), and the other after leaving it sealed in a dark cupboard for fivedays (day 5). The following results were obtained for the titration of the samples with0.0100 moldm-3 Na2S203(aq).SampleDay ODay 5Titre I cm’ ±O.I cm'(a)0)25.820.1Determine the mole ratio of S2032- to 02, using the balanced equations.(ii)(iii)Calculate the number of moles of oxygen in the day O sample.The day 5 sample contained 5.03 x 10-5 moles of oxygen.Determine the 5-day biochemical oxygen demand of the pond, in mg dm-3(“parts per million”, ppm).[2][2]0)(ii)Calculate the percentage uncertainty of the day 5 titre.Suggest a modification to the procedure that would make the results more reliable. [1]

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