$50 \ cm^{3}$ of $0.04 \ M \ K_{2}Cr_{2}O_{7}$ in acidic medium oxidizes a sample of $H_{2}S$ gas to sulphur. The volume of $0.03 \ M \ KMnO_{4}$ required to oxidize the same amount of $H_{2}S$ gas to sulphur in acidic medium is: (in $cm^{3}$)

When $KMnO_4$ solution is titrated with a solution containing $Fe^{2+}$ ions,the indicator used in this titration is

Justify that the following reactions are redox reactions:
$(a) \ CuO_{(s)} + H_{2(g)} \to Cu_{(s)} + H_2O_{(g)}$
$(b) \ Fe_2O_{3(s)} + 3CO_{(g)} \to 2Fe_{(s)} + 3CO_{2(g)}$
$(c) \ 4BCl_{3(g)} + 3LiAlH_{4(s)} \to 2B_2H_{6(g)} + 3LiCl_{(s)} + 3AlCl_{3(s)}$
$(d) \ 2K_{(s)} + F_{2(g)} \to 2K^+F^-_{(s)}$
$(e) \ 4NH_{3(g)} + 5O_{2(g)} \to 4NO_{(g)} + 6H_2O_{(g)}$

$100 \, mL$ of a sample containing calcium oxalate $[CaC_2O_4]$ is dissolved in an acidic solution. It required $20 \, mL$ of $1 \, M \, KMnO_4$ for complete oxidation. Find the mass of $Ca^{2+}$ ions (in grams) present in the sample. [Atomic mass of $Ca = 40$]

The redox reaction among the following is

$A$ copper coin is completely covered with a gold film and is placed in dilute $HNO_3$. This will result in the formation of: