Question

a new element has two isotopes, one with a mass of 40.0 amu and the other with a mass of 42.0 amu. the atomic weight is 40.50 g/mol. pick the true statement a. the isotope at 42.0 amu is less abundant b. the element has 19 protons and 21 neutrons c. the isotope at 40.0 amu is less abundant d. both isotopes are equally abundant e. the element has 20 protons and 21 electrons f. there is not enough information given g. the element is calcium

Answer 1

Statement A, "the isotope at 42.0 amu is less abundant", is true. This is because atomic weight is calculated using the relative abundances of the isotopes. Since the calculated weight is closer to the 40.0 amu isotope, this means that it must be more abundant.
Although the number of protons and neutrons add up, this statement is not necessarily true just because we know the atomic mass.