balance equation : PCl5 + 4H2O → H3PO4 + HCl

fist look at the first molecule on the left of the equation, the first atom is P there's 1 mole and looking at the right there is also only one. the look at the next atom Cl which has five mole, on the right there is only one so we put a 5 so we put a 5 in front of the HCl to balance.

so. PCl₅ + H₂O → H₃PO₄ + 5HCl

next molecule we look at H on the left there is two and on the right there is 3 + 5 so 8 all together.  so make there be 8 on the right we out a 4 in front of the H₂O. 

so PCl₅ + 4H₂O → H₃PO₄ + 5HCl

finally look at O, there is 4 on the left and 4 on the right so that is balanced,

double check to make sure there is the same mole for every atom on both side, the left and right.  

so 1P, 5Cl, 8H and 4O and both side so the equation is balanced