What is the Standard cell potential of a cell constituted by two...

The standard reduction potential of electrode B is higher than that of electrode A. Thus, electrode B acts as the cathode and electrode A acts as the anode.

The standard cell potential is given by:

$E^0_{\text{cell}} = E^0_{\text{cathode}} - E^0_{\text{anode}}$

Substituting the given values:

$E^0_{\text{cell}} = 0.42\ \text{V} - 0.30\ \text{V} = 0.12\ \text{V}$

Answer: $0.12\ \text{V}$