In the electrolysis of water, one faraday of electrical energy would liberate

According to the definition 1 F or 96500 C is the charge carried by 1 mol of electrons when water is electrolysed
$2H_{2}O\to 4H^{+}+O_2+4e^{-}$
So, 4 Faraday of electricity liberate = 32 g of O₂.
Thus 1 Faraday of electricity liberate

$=\frac{32}{4}g of O_2=8g of O_2$