How to calculate relitive molecular polarity of a large molecule with electro negitivity?

By doing the vector sum of the polar bonds.

Explanation:

The relative polarity of a molecule is the vector sum of the polarities of the bonds in the molecule. Thus, in large molecules, first, we need to identify which of the bonds are polar (with elements with a huge difference of electronegativity), and them calculate the polarity in each polar bond which will be the difference in the electronegativities.

Then, we identify the vector, which will be directed to the most electronegative element. Thus, the vectors must be summed following the rules of the vector sum.