Question

A generic element, X, is composed of two isotopes, ^40X and ^42X. ^40X has a natural abundance of 75% and an isotopic mass of 39.96 amu, and ^42X has a natural abundance of 25% and an isotopic mass of 41.96 amu. What is the average atomic mass of this element?

a) 40.96 amu
b) 40.00 amu
c) 41.00 amu
d) 41.50 amu

Answer 1

The average atomic mass of element X, considering the natural abundances and isotopic masses of its isotopes ^40X and ^42X, is calculated to be ~40.46 amu, which after rounding, matches most closely with Option B: 40.00 amu.

Step-by-step explanation:

To calculate the average atomic mass of an element with multiple isotopes, we use the natural abundances and isotopic masses of each isotope. The average atomic mass is a weighted average, meaning that each isotope contributes to the atomic mass in proportion to its natural abundance.

For the element X, which has two isotopes ^40X and ^42X:

• The contribution of ^40X to the average atomic mass is (0.75 × 39.96 amu) = 29.97 amu.
• The contribution of ^42X to the average atomic mass is (0.25 × 41.96 amu) = 10.49 amu.

Adding these together:

29.97 amu + 10.49 amu = 40.46 amu

This value doesn't exactly match any of the provided multiple-choice options, indicating a possible discrepancy. However, if we consider standard rounding conventions, the closest answer choice to 40.46 amu would be Option B: 40.00 amu.