The average atomic mass : 15.86
Further explanation
Given
205 atoms = 16 amu⇒atom 1
15 atoms = 14 amu⇒atom 2
Required
The average atomic mass
Solution
Total atoms for element Z :
= 205 + 15
= 220 atoms
% atom 1 = 205/220 x 100% = 93.18%
% atom 2 = 15/220 x 100% = 6.82%
The average atomic mass :
= 93.18% x 16 amu + 6.82% x 14 amu
= 14.9088 + 0.9548
= 15.8636≈ 15.86( 4 sig fig)