$ N_2O_4 \rightleftharpoons 2\: NO_2\: is\: \alpha.$

(g) (g)

If initial $N_2O_4(g)$is one mole, the total number of moles at equilibrium is

$\begin {array} {1 1} (1)\;1+2 \alpha & \quad (2)\;1-2\alpha \\ (3)\;1+ \alpha & \quad (4)\;1-\alpha \end {array}$