This is a numerical problem on electrolysis. It was attempted by many candidates. However, many candidates failed to convert the given units to S. I. units [time in minutes to seconds]; also they could not successfully manipulate the standard form of the given electrochemical equivalent in solving the arithmetic. The expected answer is
Let volume of oxygen at stp = V
V = mass cm, of oxygen of stp
density(ρ) of Oxygen at stp
mass liberated = zIt
volume liberated = mass
density
= 8.30 x 10-8 x 3.00 x 20 x 60
1.33
= 2.25 x 10-4 m3 =225 cm3 ) 
