Question

Why is the average atomic mass used when calculating grams of a substance?

Answer 1

The elements in the substance are mixtures of their isotopes.

Answer 2

The average atomic mass includes all of the isotopes of that atom which exist in nature. Almost all elements have at least two isotopes which naturally occur. 
Recall that isotopes are atoms with the same atomic number (protons), but a different number of neutrons. Since neutrons contribute the same amount of mass as a proton (1 amu), differing numbers of neutrons will change the mass of the atoms.

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