What is the four-electron reduced form of $\mathrm{O}_2$ ?

Kjeldahl's method cannot be used for the estimation of nitrogen in which compound?

Which of the following compounds is most reactive in $\mathrm{S}_{\mathrm{N}} 1$ reaction?

The coagulating power of electrolytes having ions $\mathrm{Na}^{+}, \mathrm{Al}^{3+}$ and $\mathrm{Ba}^{2+}$ for $\mathrm{As}_2 \mathrm{S}_3$ sol increases in the order