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EEJ MAIN Chemistry QUESTION #7190
Question 1
A $15\text{ mL}$ aqueous solution containing $Fe^{2+}$ ions in an acidic medium reacts completely with $20\text{ mL}$ of $0.03\text{ M}$ aqueous ${Cr_{2}O_{7}}^{2-}$. Calculate the molarity of the $Fe^{2+}$ solution (provide the result in terms of $\times 10^{-2}\text{ M}$).
  • 24✔️
  • 12
  • 4
  • 8
Correct Answer Explanation

The balanced redox equation is:

$6Fe^{2+} + Cr_{2}O_{7}^{2-} + 14H^{+} \rightarrow 6Fe^{3+} + 2Cr^{3+} + 7H_{2}O$

Using the equivalence principle ($n_{1}M_{1}V_{1} = n_{2}M_{2}V_{2}$):

  • For $Fe^{2+}$, $n$-factor is $1$.
  • For $Cr_{2}O_{7}^{2-}$, $n$-factor is $6$.

$1 \times M_{Fe} \times 15 = 6 \times 0.03 \times 20$

$15 M_{Fe} = 3.6 \Rightarrow M_{Fe} = 0.24\text{ M} = 24 \times 10^{-2}\text{ M}$.

[cite: 13]