Question

A sample of element X contains 90% X-35 atoms, 8.0% X-37 atoms, and 2.0% X-38 atoms. The average atomic mass will be closest to which value?

Answer 1

To find average atomic mass you multiply the mass of each isotope by its percentage, and then add the values up.

35 * 0.90 + 37 * 0.08 + 38 * 0.02 = 35.22

Average atomic mass closest to 35.22 amu.

Answer 2

Answer : The average atomic mass will be closest to 35 amu.

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:

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

As we are given that,

Mass of isotope X-35 = 35 amu

Percentage abundance of isotope X-35 = 90 %

Fractional abundance of isotope X-35 = 0.90

Mass of isotope X-37 = 37 amu

Percentage abundance of isotope X-37 = 8.0  %

Fractional abundance of isotope X-37 = 0.08

Mass of isotope X-38 = 38 amu

Percentage abundance of isotope X-38 = 2.0  %

Fractional abundance of isotope X-38 = 0.02

Now put all the given values in above formula, we get:

$\text{Average atomic mass of element}=\sum[(35\times 0.90)+(37\times 0.08)+(38\times 0.02)]$

$\text{Average atomic mass of element X}=35.22amu\approx 35amu$

Therefore, the average atomic mass will be closest to 35 amu.