How many electrons are involved in the following redox reaction?
$Cr_2O_7^{2-} + Fe^{2+} + C_2O_4^{2-} \to Cr^{3+} + Fe^{3+} + CO_2$ (Unbalanced)

In the reduction of dichromate by $Fe(II)$,the number of electrons involved per chromium atom is:

$PbS + 4O_3 \,\to ?\,+\,4O_2$

In acidic medium,$K_2Cr_2O_7$ shows oxidising action as represented in the half reaction:
$Cr_2O_7^{2-} + XH^+ + Ye^- \rightarrow 2A + ZH_2O$
$X, Y, Z$ and $A$ are respectively:

Balance the following redox reactions by the ion-electron method:
$(a)$ $MnO_4^-(aq) + I^{-}(aq) \rightarrow MnO_2(s) + I_2(s)$ (in basic medium)
$(b)$ $MnO_4^-(aq) + SO_2(g) \rightarrow Mn^{2+}(aq) + HSO_4^-(aq)$ (in acidic solution)
$(c)$ $H_2O_2(aq) + Fe^{2+}(aq) \rightarrow Fe^{3+}(aq) + H_2O(l)$ (in acidic solution)
$(d)$ $Cr_2O_7^{2-} + SO_2(g) \rightarrow Cr^{3+}(aq) + SO_4^{2-}(aq)$ (in acidic solution)

Assign $A, B, C, D$ from the given type of reaction:
$5CO(g) + I_2O_5(s) \longrightarrow 5CO_2(g) + I_2(s)$