This formula equation is unbalanced.

P4 (s) + Cl2 (g) PCl3 (l)

Which coefficient should be placed in front of PCl3 to balance this equation?

124 6

In order to balance this, you have to count each element where the elements in the reactants side and the product side should have equal number of molecules. The balanced reaction is as follows:

P4 (s) + 6Cl2 (g) = 4PCl3 (l)

Thus, the answer is 4.