108g of silver (molar mass 108g mol ^ - 1 ) is deposited at cathode...

${\left({n}_{Ag}\right)}_{deposit} = \dfrac{108}{108} = 1mole$
${Ag}^{+} + {e}^{-} ⟶ Ag$
$1F$ charge is required to deposit $1mole$ of $Ag$
${H}_{2}O$ $⟶ \dfrac{1}{2}{O}_{2} + 2{H}^{+} + 2{e}^{-}$
$2F$ charge is required to deposit $\dfrac{1}{2}mole$
$1F$ charge will deposit $\dfrac{1}{4}mole$ of ${O}_{2}$ gas.
${V}_{{O}_{2}} = \dfrac{nRT}{P}$
$= \dfrac{1}{4} \times \dfrac{0.0821 \times 273}{1} = \dfrac{1}{4} \times 22.4$
${V}_{{O}_{2}} = 5.6L$

Faraday's second law of electrolysis states that, when the same quantity of electricity is passed through several electrolytes, the mass of the substances deposited are proportional to their respective equivalent weight.