Question

Both PCl5 and NH3 are symmetrical but dipole moment for ammonia is not zero. Why

Answer 1

PCl5 has a symmetrical geometry, the vector sum of the dipole moments of the different P-Cl bonds cancel each other. Therefore, the overall dipole moment of PCl5 becomes 0.
Whereas, in the case NH3 then geometry isn't symmetrical as one of the corners of the tetrahedron shape is occupied by a LONE PAIR  while the others are occupied by hydrogens. In this case vector sum of different dipole moment is not equal to 0 .