A 100.0-mL sample of spring water was treated to convert any iron present to Fe2+. Addition of 25.00 mL of 0.002107 M K2Cr2O7 resulted in the reaction: 6 Fe2+ + Cr2O72- + 14 H+ ---> 6 Fe3+ + 2 Cr3+ + 7 H2O. The excess K2Cr2O7 was back-titrated with 7.47 mL of a 0.00979 M Fe+2 solution. Calculate the parts per million of iron in the sample.