Answer
Mole ratio of $Cl_{2(g)}: O_{2(g)} =2: 7$
Work Step by Step
Dissociation reactions can be written as,
$ Cl_{2}O_{7(g)} → 2 Cl_{(g)} + 7 O{(g)}$
Formation of molecular chlorine and oxygen can be written as,
$ 2 Cl{(g)} → Cl_{2(g)} $
$ 7O_{(g)} → 3.5 O{2(g)}$
From this we have the ratio of number of moles of $Cl_{2(g)}: O{2(g)} $ is 1: 3.5, or $2: 7$
