Question

The astronaut then measures the abundance of magnesium on the new planet, obtaining the following results:

Answer 1

Solution:

Atomic mass = ΣzₓAₓ  zₓ = relative abundance of the isotope
and Aₓ = the atomic mass of the isotope.

Atomic mass = (0.0946x85.91) + (0.07x86.91) + (0.8354x87.91)
= 87.65 amu

Answer 2

Answer: The average atomic mass of Strontium is 87.65 u

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:

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

• For $_{38}^{86}\textrm{Sr}$ isotope:

Mass of $_{38}^{86}\textrm{Sr}$ isotope = 85.91 u

Percentage abundance of $_{38}^{86}\textrm{Sr}$ isotope = 9.46 %

Fractional abundance of $_{38}^{86}\textrm{Sr}$ isotope = 0.09460

• For $_{38}^{87}\textrm{Sr}$ isotope:

Mass of $_{38}^{87}\textrm{Sr}$ isotope = 86.91 u

Percentage abundance of $_{38}^{87}\textrm{Sr}$ isotope = 7.00 %

Fractional abundance of $_{38}^{87}\textrm{Sr}$ isotope = 0.0700

• For $_{38}^{88}\textrm{Sr}$ isotope:

Mass of $_{38}^{88}\textrm{Sr}$ isotope = 87.91 u

Percentage abundance of $_{38}^{88}\textrm{Sr}$ isotope = 83.54 %

Fractional abundance of $_{38}^{88}\textrm{Sr}$ isotope = 0.8354

Putting values in equation 1, we get:

$\text{Average atomic mass of Sr}=[(85.91\times 0.0946)+(86.91\times 0.0700)+(87.91\times 0.8354)]$

$\text{Average atomic mass of Sr}=87.65u$

Hence, the average atomic mass of Strontium is 87.65 u