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

In an experiment,$50 \ mL$ of $0.1 \ M$ solution of a metal salt reacted with $25 \ mL$ of $0.1 \ M$ solution of sodium sulphite. The half-equation for the oxidation of sulphite ion is $SO_3^{2-}{(aq)} + H_2O_{(l)} \to SO_4^{2-}{(aq)} + 2H^{+}{(aq)} + 2e^-$. If the oxidation number of the metal in the salt was $3$,what would be the new oxidation number of the metal?

Explain why the colour of $KMnO_4$ disappears when oxalic acid is added to its solution in acidic medium.

$6 Fe_3O_4 + 2 MnO_4^- + H_2O \to 9 Fe_2O_3 + 2 MnO_2 + 2 OH^-$ Calculate the equivalent weight of $Fe_3O_4$ in the above equation.

Assign $A, B, C, D$ from the given type of reaction.
$Fe^{2+}(aq.) + Cr_2O_7^{2-}(aq.) + H^+(aq.) \longrightarrow Fe^{3+}(aq.) + Cr^{3+}(aq.) + H_2O(l)$

Assign $A, B, C, D$ from the given type of reaction.
$NH_4Cl + NaNO_2 \xrightarrow{\Delta} N_2 \uparrow + NaCl + 2H_2O$