Final answer:
To calculate the average mass of a Mg atom, multiply the mass of each isotope by its abundance and sum them up.
Step-by-step explanation:
To calculate the average mass of a Mg atom, we need to consider the mass of each isotope of magnesium and its abundance. We can use the formula:
Average mass = (mass of isotope 1 x abundance of isotope 1) + (mass of isotope 2 x abundance of isotope 2) + (mass of isotope 3 x abundance of isotope 3)
Applying the formula using the given values:
Average mass = (23.98 amu x 0.787) + (24.99 amu x 0.1013) + (25.98 amu x 0.1117) = 24.31 amu