An unknown element has two isotopes: one whose mass is 68.926 amu (60.00% abundance) and the other whose mass is 70.925 amu (40.00% abundance). The average atomic mass of the element is equal to .

69.726 amu

Explanation:

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

Isotope A:

Mass of A = 68.926 amu

Abundance of A (A%) = 60.00%

Isotope B:

Mass of B = 70.925 amu

Abundance of B (B%) = 40.00%

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]

= [(68.926 × 60)/100] + [(70.925 × 40)/100]

= 41.3556 + 28.37

= 69.726 amu

Therefore, the average atomic mass of the element is 69.726 amu