If  9g of acidulated $H_{2}O$ is electrolysed completely with the current of 50% efficiency then:

Equivalent mass of  $H_{2}O=9g$
$\therefore$  9g of  $H_{2}O$  will be electrolyzed by a 2 faraday (96500C) if the efficiency is 50%.
During this electrolysis; 5.6L  $O_2$  will be liberated at STP because the equivalent volume of O2 is 5.6 at STP.