Answer : The mole ratio of \(PCl_3\) to \(PCl_5\) is 1 : 1 ratio.
Explanation :
The balanced reaction will be,
\(PCl_3+Cl_2\rightarrow PCl_5\)
From the balanced reaction, we conclude that
1 mole of \(PCl_3\) react with the 1 mole of \(Cl_2\) to give 1 mole of \(PCl_5\)
Hence, the mole ratio of \(PCl_3\) to \(PCl_5\) is 1 : 1 ratio.