Question

An element has two isotopes
having atomic
mass 10 and 15 y,
respectively. If the %
abundance of lighter isotope is 80% then the
average atomic mass of the element is
(1) 9u

(2) 11 u
(3) 12 u
(4) 14 u​

Answer 1

Answer:

(3) 11 u

Explanation:

according to question

equation = 80÷100×10+20÷100×15 = 11u

Answer 2

Given:

Mass of isotope 1= 10u

Mass of isotope 2= 15u

Percentage abundance of isotope 1= 80%

To find:

The average atomic mass of the element

Solution:

When an element exists in nature in more than one isotopic form, instead of using absolute atomic mass we use the average atomic mass of the element.

Given, percentage abundance of isotope 1= 80%

Hence, percentage abundance of isotope 2= 100-80= 20%

Average atomic mass of element=

∑(mass of isotope X % abundance of isotope) / 100

Substituting values,

Average atomic mass= $\frac{(10u X 80) + (15u X 20)}{100}$

= $\frac{800u + 300u}{100}$ = 11u

Hence, the  average atomic mass of the element is 11u.