Question

What is the voltage of a galvanic cell made with zinc (zn) and aluminum (al)?

Answer 1

From the reduction standard potentials;
The emf of Zinc = -0.76 V
and the emf of Aluminium = -1.66 V
In a galvanic cell the component with lower standard reduction potential gets oxidized and that it is added to the anode compartment. 
Therefore. the voltage of a galvanic cell made with zinc and aluminium will be;
Voltage =Ered- Eoxd
             = -0.76 - (-1.66)
             = 0.9 V

Answer 2

Answer : The voltage of galvanic cell made with zinc and aluminum is, 0.90V

Explanation :

We are taking the value of standard reduction potential form the standard table.

$E^0_{[Al^{3+}/Al]}=-1.66V$

$E^0_{[Zn^{2+}/Zn]}=-0.76V$

In this cell, the component that has lower standard reduction potential gets oxidized and that is added to the anode compartment. The second forms the cathode compartment.

The half oxidation-reduction reaction will be :

Oxidation : $Al+3e^-\rightarrow Al^{3+}$       $E^0_{[Al/Al^{3+}]}=1.66V$

Reduction : $Zn^{2+}+2e^-\rightarrow Zn$       $E^0_{[Zn^{2+}/Zn]}=-0.76V$

The expression for standard cell is,

$E^0{cell}=E^0{Anode}+E^0{Cathode}$

$E^0=E^0_{[Al/Al^{3+}}+E^0_{[Zn^{2+}/Zn]}$

$E^0=1.66V+(-0.76V)$

$E^0=0.90V$

Therefore, the voltage of galvanic cell made with zinc and aluminum is, 0.90 V