You are given a 200-mL sample of surface water from Lake Ontario, and asked to determine the chemical oxygen demand (COD) of the sample. To do so, you reflux the sample with 15 mL of a 0.2M K2Cr2O7 standard solution for 2 hours, then titrate the excess dichromate using 20.0 mL of a 0.3M Fe2+ solution. What was the COD of the sample (reported in mol/L of O2)?
Reminder: Cr2O72- consumes 6e-, Fe2+ offers 1e-.
A. The COD of the sample is 0.010 mol/L O2
B. The COD of the sample is 0.008 mol/L O2
C. The COD of the sample is 0.015 mol/L O2
D. The COD of the sample is 0.005 mol/L O2