What is the number of moles of electrons gained by one mole oxidizing agent in following redox reaction?

$$\mathrm{Zn_{(s)}+2HCl_{(aq)}}$$ $$\longrightarrow$$ $$\mathrm{ZnCl_2+H_2}$$

Which from following elements exhibits usual tendency to undergo reduction?

What is the change in oxidation number of selenium in the following redox reaction?

$$\mathrm{SeO}_{3(\text { (a) })}^{2-}+\mathrm{Cl}_{2(\mathrm{~g})}+2 \mathrm{OH}^{-} \longrightarrow \mathrm{SeO}_{4(\text { (a) })}^{2-}+2 \mathrm{Cl}_{(\text {(a) })}^{-}+\mathrm{H}_2 \mathrm{O}$$

What is the change in oxidation number of $$\mathrm{Cr}$$ in the following redox reaction?

$$3 \mathrm{H}_2 \mathrm{O}_{2(\mathrm{aq})}+\mathrm{Cr}_2 \mathrm{O}_{7 \text { (aq) }}^{2-}+8 \mathrm{H}_{(\mathrm{aq})}^{+} \longrightarrow 3 \mathrm{O}_{2(\mathrm{~g})}+2 \mathrm{Cr}^3+7 \mathrm{H}_2 \mathrm{O}$$