Ozone is estimated in air by passing certain volume of air through an acidified or neutral or basic KI solution when O2 is evolved and I⊝ is oxidised to I2. Free I2 evolved is titrated with standard Na2S2O3 solution. In an experiment, 10 L of air at 1 atm and 27oC was passed through an alkaline KI solution and I2 liberated required 2 mL of M800 Na2S2O3 solution. The volume percentage of O3 in sample is :