Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$FeSO_4 + Na_2S \longrightarrow FeS \downarrow$

Copper in an alloy is estimated by dissolving in conc. nitric acid. In this process,copper is converted to cupric nitrate with the evolution of nitric oxide $(NO)$. The mixture when treated with potassium iodide forms cupric iodide,which is unstable and decomposes to cuprous iodide and iodine. The amount of copper in the alloy is estimated by titrating the liberated iodine with sodium thiosulphate. The reactions are:
$a \,Cu + b \,HNO_3 \rightarrow c \,Cu(NO_3)_2 + d \,NO + e \,H_2O$
$f \,CuI_2 \rightarrow g \,Cu_2I_2 + h \,I_2$
$i \,Na_2S_2O_3 + j \,I_2 \rightarrow k \,Na_2S_4O_6 + l \,NaI$
(Fill in the blanks)
$(a)$ The coefficients are: $a=\ldots, b=\ldots, c=\ldots, d=\ldots$ and $e=\ldots$.
$(b)$ The coefficients are: $f=\ldots, g=\ldots$ and $h=\ldots$.
$(c)$ The coefficients are: $i=\ldots, j=\ldots, k=\ldots$ and $l=\ldots$.
$(d)$ If $2.54 \,g$ of $I_2$ is evolved from a $2.0 \,g$ sample of the alloy,what is the percentage of copper in the alloy? (Atomic weights of iodine and copper are $127$ and $63.5$,respectively).

$A$ copper coin is completely covered with a gold film and is placed in dilute $HNO_3$. This will result in the formation of:

Balance the following redox reactions using the oxidation number and ion-electron method:
$(1) \ Zn + NO_3^- + H^+ \to Zn^{2+} + N_2O + H_2O$
$(2) \ I^- + O_2 + H_2O \to I_2 + OH^-$
$(3) \ MnO_4^- + C_2H_5OH \to Mn^{2+} + CH_3COOH$

In the reaction $I_2 + 2S_2O_3^{2-} \to 2I^{-} + S_4O_6^{2-}$,the equivalent weight of iodine will be equal to

Which one of the following conversions does not involve either oxidation or reduction?