A 100mL sample of water was treated to convert any iron present to ${\mathrm{Fe}}^{2+}$ 25mL of $0.002 {\mathrm{M} \mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$

resulted in the reaction.

 $6{\mathrm{Fe}}^{2+}+{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+14{\mathrm{H}}^{+}⟶6{\mathrm{Fe}}^{3+}+2{\mathrm{Cr}}^{3+}+7{\mathrm{H}}_2\mathrm{O}$

The excess ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ was back-titrated with $7.5 \mathrm{mL}$ of $0.01 \mathrm{M} {\mathrm{Fe}}^2$ solution. Calculate the parts per million (ppm) of iron in the water sample.