First find E vector through [kp{1+3cos^2x}^1\2]/r^3 (r=distance from middle point of dipole to the treated charge i.e. Cl ion.) For your case it will reduced as [2kp/r^3] and then use Electric Force= Charge * Electric Field, hope this will derive the proper answer. [For P (Charge*Distance).] In...
I would like to add one thing here, in practical applications normally I consider "Clockwise" and "Anticlockwise" notation for torque are more useful. You can imagine a hanging pendulum and if you are forcing it in a way that it tends the pendulum moving in AntiClockWise direction, the torque...