Question

why is the average atomic mass used when calculating grams of a substance?
a. the elements in the substance are mixtures of their isotopes.
b. the number of isotopes in the sample is constantly changing.
c. the exact weights of the isotopes are not known.
d. the average atomic mass is known to more significant figures.

Explanation:

Elements in nature are often mixtures of their isotopes. Each isotope has a different mass. The average atomic mass takes into account the relative abundances of these isotopes to give a representative mass for calculations.

Answer:

A. The elements in the substance are mixtures of their isotopes.