Question

A hypothetical element consists of two isotopes of masses 69.95 amu and 71.95 amu with abundances of 25.7% and 74.3%, respectively. What is the average atomic mass of this element?

a. 71.95 amu
b 71.4 amu
c. 69.95 amu
d. 70.95 amu
e. 70.5 amu

Answer 1

Option B. 71.4 amu

Step-by-step explanation:

From the question given above, the following data were obtained:

Isotope A:

Mass of A = 69.95 amu

Abundance (A%) = 25.7%

Isotope B:

Mass of B = 71.95 amu

Abundance (B%) = 74.3%

Average atomic mass =..?

The average atomic mass of the element can be obtained as shown below:

Average atomic mass = [(Mass of A × A%)/100)] + [(Mass of B × B%)/100)]

= [(69.95 × 25.7)/100] + [(71.95 × 74.3)/100]

= 17.977 + 53.459

= 71.436 ≈ 71.4 amu

Therefore, the average atomic mass of the element is 71.4 amu