Question

. Naturally occurring element X exists in three isotopic forms: X-28 (27.979 amu, 92.21% abundance), X-29 (28.976 amu 4.70% abundance), and X-30 (29.974 amu 3.09% abundance). Calculate the atomic weight of X. a. 29.09 amu b. 28.09 amu c. 35.29 amu d. 86.93 amu e. 25.80 amu

Answer 1

Answer:

The correct answer is option b.

Explanation:

Average atomic mass of an element is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.

Formula used to calculate average atomic mass follows:

$\text{Average atomic mass }=\sum_{i=1}^n\text{(Atomic mass of an isotopes)}_i\times \text{(Fractional abundance})_i$ .....(1)

We are given:

Mass of isotope X-28 = 27.979 amu

Percentage abundance of isotope X-28 = 92.21%

Fractional abundance of isotope X-28 = 0.9221

Mass of isotope X-29 = 28.976 amu

Percentage abundance of isotope X-29 = 4.70%

Fractional abundance of isotope X-29 = 0.047

Mass of isotope X-30= 29.974 amu

Percentage abundance of isotope X-30 = 3.09%

Fractional abundance of isotope X-30 = 0.0309

Putting values in equation 1, we get:

$\text{Average atomic mass of X}=\sum[(27.979 amu\times 0.9221)+(28.976 amu\times 0.047)+(29.974 amu\times 0.0309)]$

$\text{Average atomic mass of X}=28.09 amu$

The atomic weight of X is 28.09 amu.

Answer 2

Answer: b 28.09 amu

The atomic weight A W = (isotopic contribution *relative abundance) /sum of relative abundance

A. W. = (29.979*92.21+28.976*4.7+29.974*3.09)/92.21+4.7+3.09

A. W. = 2808.75045/100

A. W. = 28.0875 = 28.09amu

The Atomic Weight = 28.09amu