SOLUTION

Number of moles of water= 9 /18=0.5 

Balanced equation for electrolysis of water

2H₂O?2H₂+O₂ 

∴2 moles of H₂O dissociates to produce 1 mole of O₂ at STP. 

∴0.5 moles of H₂O produces 0.250.25 moles of O₂ at STP.

∴0.25 moles of O₂ will occupy= 0.25×22.4 = 5.6 litres