100 mL of aqueous solution of $0.05 \mathrm{M} \mathrm{Cu}^{2+}$ is added to 1 L of 0.1 M KI solution. The resultant solution was titrated with $0.01 \mathrm{M} \mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$ solution using starch indicator till blue colour disappeared. What is the volume (in mL ) of $\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$ used?

Identify the correct statements about the following stoichiometric equation.

$$ a \mathrm{P}_4+b^{-} \mathrm{OH}+c \mathrm{H}_2 \mathrm{O} \longrightarrow d \mathrm{PH}_3+e \mathrm{H}_2 \mathrm{PO}_2^{-} $$

I. $a+b+c=5$

II. $b+c-e=3$

III. The oxidation state of P in $\mathrm{H}_2 \mathrm{PO}_2^{-}$is +l .

$\mathrm{KMnO}_4$ oxidises hydrogen sulphide in acidic medium, The number of moles of $\mathrm{KMnO}_4$ which react with one mole of hydrogen sulphide is

$\mathrm{H}_2 \mathrm{O}_2$ reduces $\mathrm{KMnO}_4$ in acidic medium to ' $x$ ' and in basic medium to ' $y$ '. What are $x$ and $y$ ?