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EEJ MAIN Chemistry QUESTION #7497
Question 1

In which of the following processes is the change in oxidation state equal to five?

  • $\text{C}_2\text{O}_4^{2-} \to 2\text{CO}_2$
  • $\text{MnO}_4^- \to \text{Mn}^{2+}$✔️
  • $\text{Cr}_2\text{O}_7^{2-} \to 2\text{Cr}^{3+}$
  • $\text{CrO}_4^{2-} \to \text{Cr}^{3+}$
Correct Answer Explanation

Calculate the change in oxidation state for each:

  • (A) C in C$_2$O$_4^{2-}$: $+3$; C in CO$_2$: $+4$. Change $= +1$ per C atom (2 C, so total = 2, but per atom = 1).
  • (B) Mn in MnO$_4^-$: $+7$; Mn in Mn$^{2+}$: $+2$. Change $= 7 - 2 = \mathbf{5}$ ✓
  • (C) Cr in Cr$_2$O$_7^{2-}$: $+6$; Cr in Cr$^{3+}$: $+3$. Change $= 3$ per Cr atom.
  • (D) Cr in CrO$_4^{2-}$: $+6$; Cr in Cr$^{3+}$: $+3$. Change $= 3$.

The change of 5 occurs in the conversion $\text{MnO}_4^- \to \text{Mn}^{2+}$.