To an acidified dichromate solution,a pinch of $Na_2O_2$ is added and shaken. What is observed?

Suggest a scheme of classification of the following redox reactions:
$(a) N_{2(g)} + O_{2(g)} \rightarrow 2NO_{(g)}$
$(b) 2Pb(NO_3)_{2(s)} \rightarrow 2PbO_{(s)} + 4NO_{2(g)} + O_{2(g)}$
$(c) NaH_{(s)} + H_2O_{(l)} \rightarrow NaOH_{(aq)} + H_{2(g)}$
$(d) 2NO_{2(g)} + 2OH^{-}_{(aq)} \rightarrow NO_2^-(aq) + NO_3^-(aq) + H_2O_{(l)}$

$50 \ cm^{3}$ of $0.04 \ M \ K_{2}Cr_{2}O_{7}$ in acidic medium oxidizes a sample of $H_{2}S$ gas to sulphur. The volume of $0.03 \ M \ KMnO_{4}$ required to oxidize the same amount of $H_{2}S$ gas to sulphur in acidic medium is: (in $cm^{3}$)

Which of the following can act both as an oxidizing and a reducing agent?

In basic medium,$CrO_{4}^{2-}$ oxidises $S_{2}O_{3}^{2-}$ to form $SO_{4}^{2-}$ and itself changes into $Cr(OH)_{4}^{-}$. The volume of $0.154 \ M \ CrO_{4}^{2-}$ required to react with $40 \ mL$ of $0.25 \ M \ S_{2}O_{3}^{2-}$ is ........... $mL$ (Rounded-off to the nearest integer).

Assign $A, B, C, D$ from given type of reaction.
$2KI + CuSO_4 \longrightarrow CuI \downarrow + \frac{1}{2}I_2 + K_2SO_4$