One litre of oxygen at a pressure of 1 atm. and two litres of nitrogen at a pressure of $0.5\;atm.$ are introduced into a vessel of volume 1 litre. If there is no change in temperature, the final pressure of the mixture of gas (in atm) is

Question

\[\begin {array} {1 1} (1)\;1.5 & \quad (2)\;2.5 \\ (3)\;2 & \quad (4)\;4 \end {array}\]