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

answer: c. the experiment shows that the red substance experienced a chemical change.

explanation: chemical change is the change in which chemical composition of the substance changes. physical change is the change in which there is no change in chemical composition.

chemical changes are accompanied by following changes:

a) precipitate formation

b) energy is released or absorbed

c) formation of bubbles

d) change in color

as it is given that the red color has disappeared and there is production of gas, it means the red substance has undergone a chemical change.

e

clouds form

solid ice forms on leaves.

water droplets form on grass.