How many electrons are needed to reduce $N_2$ to $NH_3$?

When $4$ moles of electrons are added to $NO_3^-$,the product obtained is .......

After balancing the equation,$C_2O_4^{2-} + H^+ + MnO_4^- \longrightarrow CO_2 + Mn^{2+} + H_2O$,the coefficient of $CO_2$ is:

The reaction of white phosphorus on boiling with alkali in an inert atmosphere results in the formation of product '$A$'. The reaction of $1 \ mol$ of '$A$' with an excess of $AgNO_3$ in an aqueous medium gives ............... $mol(s)$ of $Ag$. (Round off to the Nearest Integer).

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$2NaI + MnO_2 + 2H_2SO_4 \to MnSO_4 + Na_2SO_4 + I_2 + 2H_2O$

In alkaline medium,the reduction of permanganate anion involves a gain of $........$ electrons.