Please refer to the attached file.
The center of mass of a polar covalent molecule or bond is meaningless but the center of the positive charge and the center of the negative charge mean a lot.
For nonpolar molecules, e.g., CH4, the center of the positive charge and the center of the negative charge are the same location (at the carbon atom).
To define polar covalent bond, the electronegativity difference between the two atoms should be around 0.5 to 2.0. HF will be an example of polar covalent bond (ΔEN = 4.0 - 2.1 = ...
This solution is provided in 376 words in an attached .doc file. It discusses how to find the center of the charge of the molecule, as well as how to calculate center of mass. A diagram is provided to help understanding of the problem.