What is $emf$?

When a rod of metal $A$ is dipped in an aqueous solution of metal $B$ (concentration of $B^{2+}$ ion being $1 \ M$) at $25 \ ^oC$,the standard electrode potentials are $E^o_{A^{2+}/A} = -0.76 \ V$ and $E^o_{B^{2+}/B} = +0.34 \ V$. What will happen?

The standard reduction electrode potentials of four elements are:
$A = -0.250 \ V$,$B = -0.136 \ V$,$C = -0.126 \ V$,$D = -0.402 \ V$
The element that displaces $A$ from its compounds is:

$Mg^{2+}$ displaces hydrogen from acids but copper does not. $A$ galvanic cell prepared by combining $Cu/Cu^{2+}$ and $Mg/Mg^{2+}$ has an $EMF$ of $2.71 \ V$ at $298 \ K$. If the potential of copper electrode is $0.34 \ V$,what is the reduction potential of $Mg$ electrode?

Give the cell reaction and $E_{cell}^o$ value for the cell constructed using the given standard electrode potentials: $E_{(H^+|O_2|H_2O)}^o = 1.23 \ V$ and $E_{(Fe^{2+}|Fe)}^o = -0.44 \ V$.

The standard potential of a $Co^{2+} | Co$ electrode is $-0.28 \ V$ and the standard potential of the cell $Pt | Ti^{2+}(aq.), Ti^{3+}(aq.) || Co^{2+}(aq.) | Co_{(s)}$ is $0.09 \ V$. What is the standard potential of the $Ti^{2+}(aq.) | Ti^{3+}(aq.)$ electrode (in $V$)?