A barometer tube is 1 m long and `2cm^(2)` in cross section. Mercury stands to a height of `75cm` in the tube. When a small amount of oxygen is introduced in the space above the mercury level, the level falls by 5cm. Calculate the mass of the oxygen is introduced. Room temperature=`27^(@)C`, g=10 m s_(-2)` and density if mercury =`13600kg m^(-3)`.