15 mL of aqueous solution of $$Fe^{2+}$$ in acidic medium completely reacted with 20 mL of 0.03 M aqueous $$Cr_2O_7^{2-}$$. The molarity of the $$Fe^{2+}$$ solution is ________ $$\times 10^{-2}$$ M. (Round off to the Nearest Integer).

In acidic medium, dichromate oxidizes ferrous ions according to the balanced equation: $$Cr_2O_7^{2-} + 6Fe^{2+} + 14H^+ \to 2Cr^{3+} + 6Fe^{3+} + 7H_2O$$. This shows that 1 mole of $$Cr_2O_7^{2-}$$ reacts with 6 moles of $$Fe^{2+}$$.

The moles of $$Cr_2O_7^{2-}$$ used = $$20 \times 10^{-3} \times 0.03 = 6 \times 10^{-4}$$ mol. From the stoichiometry, moles of $$Fe^{2+}$$ = $$6 \times 6 \times 10^{-4} = 36 \times 10^{-4}$$ mol.

The molarity of the $$Fe^{2+}$$ solution is $$\frac{36 \times 10^{-4}}{15 \times 10^{-3}} = \frac{36}{15} \times 10^{-1} = 2.4 \times 10^{-1} = 0.24$$ M. Expressed as $$\_\_ \times 10^{-2}$$ M, this is $$24 \times 10^{-2}$$ M. The answer is 24.