Observe the following reactions

(i) $2 \mathrm{KClO}_3(s) \xrightarrow{\Delta} 2 \mathrm{KCl}(s)+3 \mathrm{O}_2(g)$

(ii) $2 \mathrm{H}_2 \mathrm{O}_2(a q) \xrightarrow{\Delta} 2 \mathrm{H}_2 \mathrm{O}(l)+\mathrm{O}_2(g)$

(iii) $\mathrm{AgNO}_3(a q)+\mathrm{KCl}(a q) \longrightarrow \mathrm{AgCl}(s)+\mathrm{KNO}_3(a q)$

(iv) $2 \mathrm{Na}(s)+\frac{1}{2} \mathrm{O}_2(g) \longrightarrow \mathrm{Na}_2 \mathrm{O}(s)$

The number of redox reactions in thsi list is

Acidification of chromate gives ' $Z$ '. The oxidation state of chromium in ' $Z$ ' is

100 mL of $0.1 \mathrm{M} \mathrm{Fe}^{2+}$ solution was titrated with $\frac{1}{60} \mathrm{M}$ $\mathrm{Cr}_2 \mathrm{O}_7^{2-}$ solution in acid medium. What is the volume (in L ) of $\mathrm{Cr}_2 \mathrm{O}_7^{2-}$ solution consumed ?

Which of the following reactions of $\mathrm{KMnO}_4$ occurs in acidic medium ?