Answer
Please see the work below.
Work Step by Step
We know that, using Avogadro's law, the mole ratio and the volume ratio of gases are the same; thus, both are diatomic molecules, and the ratio can be determined by balancing the equation:
$1Cl_2+3F_2\rightarrow 2Cl_xF_y$
Thus, by the above equation: x=1 and y=3, making it: $ClF_3.$
