Explanation:
First write down the basic chemical equation with the reactants and products,
[tex]\text{P}_4+\text{ Cl}_2\to\text{ PCl}_3[/tex]
Balancing the chemical equation*,
[tex]\text{P}_4 (s) + 6 \text{Cl}_2 (g) \to 4 \text{PCl}_3 (l)[/tex]
[tex]\color{blue}\text{Note: To balance any chemical equations, we should ensure the number of atoms of each elements are the same on both the product and reactant side.}[/tex]
[tex]\color{blue}\text{The phosphorous molecule has 4 P, but the }\text{PCl}_3\text{ molecule on the product side only has one P atom. Therefore, we need 4 }\text{PCl}_3\text{ molecules to balance.}[/tex]
[tex]\color{blue}\text{But that will mean we have }4\cdot 3=12\text{Cl atoms on the product side. So we need 6 }\text{Cl}_2 \text{ molecules on the reactant side.} [/tex]
