See

Orthaber, D.; Bergman, A.; Glatter, O. SAXS experiments on absolute scale with Kratsky systems using water as a secondary standard. J. Appl. Cryst. 2000, 33, 218-225.

Dear Ashwani!

To your second question: Does mass density and electron density has any relation?

A higher atomic mass implies in general a higher mass of atoms → a higher nuclear charge → more electrons within the atomic shell. The volume of the atoms increase or differ only little. But we have to consider also the chemical bond resp. the interactions between atoms. This results in a more or less dense package of the atoms. Physical conditions like pressure and temperature affect also the density of electrons within a certain volume of material. It should be pointed to the fact that only a part of the electrons within the atomic shell is „active“. The inner-shell electrons are inactive and mere accessible at high energies.

My Regards! Hans