The mole ratio of PCl3 to PCl5 is 1:1
Explanation
PCl3 + Cl2→ PCl5
Mole ratio - Is the ratio between amount of mole of two compound.
The mole ratio is determined by considering the coefficient in front of the formula in a balanced chemical equation.
The coefficient in front of PCl3 in equation above is 1 , while the coefficient in front of PCl5 is 1, therefore the mole ratio is 1:1
