The formula to determine the average atomic mass is:
Average atomic mass = [tex]mass of the isotope 1 \times natural abundance of isotope 1[/tex] + [tex]mass of the isotope 2 \times natural abundance of isotope 2[/tex] + ........ [tex]+ mass of the isotope n \times natural abundance of isotope n[/tex] -(1)
The atomic mass of first isotope of magnesium = 24 u (given)
The natural abundance of first isotope = 78.70 % (given)
The atomic mass of second isotope of magnesium = 25 u (given)
The natural abundance of first isotope = 10.13 % (given)
The atomic mass of third isotope of magnesium = 26 u (given)
The natural abundance of first isotope = 11.17 % (given)
Substituting the values in formula (1):
atomic weight = [tex](24 \times 0.7870) + (25 \times 0.1013) + (26 \times 0.1117)[/tex]
atomic weight = [tex]24.4625 u[/tex]
Hence, the atomic weight of naturally occurring isotopic mixture is [tex]24.4625 u[/tex].
