Which of the following acts as an oxidising agent in a hydrogen-oxygen fuel cell?

In the experiment set up for the measurement of $EMF$ of a half-cell using a reference electrode and a salt bridge,when the salt bridge is removed,the voltage

The maximum work that can be obtained from the following cell is
$X|X^{2+}_{(aq)} || Y^{+}_{(aq)}| Y$
Given,$E^{\circ}_{X^{2+}/X} = -1.7 \ V, E^{\circ}_{Y^{+}/Y} = 0.8 \ V$

$A^{2+}_{(aq.)} + 2e^- \to A_{(s)}$ ; $E^{\circ} = 0.8 \ V$
$B^{3+}_{(aq.)} + 3e^- \to B_{(s)}$ ; $E^{\circ} = 0.6 \ V$
Using the above information,find the ion which will be deposited first at the cathode if a solution containing $A^{2+}_{(aq.)}$ and $B^{3+}_{(aq.)}$ is electrolysed.

The acid used in lead storage cells is

Which is the correct symbolic representation for the following cell reaction?
$Mg_{(s)} + Cl_{2(g)} \rightarrow Mg^{2+}_{(aq)} + 2Cl^{-}_{(aq)}$