Upon the addition of a solution $A$ to a strongly acidified solution of barium nitrate,a white precipitate was obtained which did not dissolve even after large addition of water. Solution $A$ contained

Identify the substance oxidised,reduced,oxidising agent,and reducing agent for each of the following reactions:
$(a)$ $2AgBr_{(s)} + C_6H_6O_{2(aq)} \to 2Ag_{(s)} + 2HBr_{(aq)} + C_6H_4O_{2(aq)}$
$(b)$ $HCHO_{(l)} + 2[Ag(NH_3)_2]^+_{(aq)} + 3OH^-_{(aq)} \to 2Ag_{(s)} + HCOO^-_{(aq)} + 2NH_{3(aq)} + 2H_2O_{(l)}$
$(c)$ $HCHO_{(l)} + 2Cu^{2+}_{(aq)} + 5OH^-_{(aq)} \to Cu_2O_{(s)} + HCOO^-_{(aq)} + 3H_2O_{(l)}$
$(d)$ $N_2H_{4(l)} + 2H_2O_{2(l)} \to N_{2(g)} + 4H_2O_{(l)}$
$(e)$ $Pb_{(s)} + PbO_{2(s)} + 2H_2SO_{4(aq)} \to 2PbSO_{4(s)} + 2H_2O_{(l)}$

$AgCl$ on fusion with $Na_2CO_3$ forms

Iron is dropped in dilute $HNO_3$,it gives:

Which of the following will not change the orange colour of $K_2Cr_2O_7$ into green colour in an acidic medium?

In the redox reaction $2MnO_4^- + 5C_2O_4^{2-} + 16H^{+} \rightleftharpoons 2Mn^{2+} + 10CO_2 + 8H_2O$,the ion acting as autocatalyst is