The reason why the number I calculated for Avogadro’s number may match the actual number is because the comparison of the number of molecules in the top layer to the number of moles in the top layer is the definition of Avogadro’s number. It is the amount of molecules in a mole <span>of any given substance.</span>
Given,
The two isotopes of B are 10B and 11B
% abundance of 10B = 19.80
% abundance of 11B = 80.20
Average atomic mass of B
= 
= 
= 
= 
= 10.802
Therefore, the average atomic mass of B is 10.802 u