What is to be done to determine the potential of any half cell?

If the $\Delta G$ of a cell reaction $AgCl + e^- \to Ag + Cl^{-}$ is $-21.20 \ kJ$,the standard e.m.f. of the cell is ............ $V$.

The reducing ability of the metals $K$,$Au$,$Zn$ and $Pb$ follows the order

Consider the following cell:
$Zn_{(s)} \mid Zn^{2+}_{(1M)} \parallel KCl_{(sat)} \mid Hg_2Cl_{2(paste)} \mid Hg$
$E^{\circ}_{cell} = 1.007 \ V$ and $E^{\circ}_{calomel} = 0.242 \ V$
What is the standard potential of $Zn$ (in $V$)?

The standard reduction potential for $Fe^{2+}/Fe$ and $Sn^{2+}/Sn$ electrodes are $-0.44 \ V$ and $-0.14 \ V$ respectively. For the cell reaction,$Fe^{2+} + Sn \longrightarrow Fe + Sn^{2+}$,the standard emf is:

Which statement is correct for $\Delta G$ and $E_{cell}$ for a cell reaction?