Calculate Average Atomic Mass from Isotope Data

Published on March 18, 2026

Word-Problem

This problem requires calculating the weighted average atomic mass of Fictitious element Fx using the mass and abundance percentage of its two isotopes. The calculation involves summing the products of each isotope's mass and its fractional abundance.

Question

Answer

This is a problem that requires calculating the average atomic mass of an element given the masses and natural abundances of its isotopes.

Here is the step-by-step solution:

1. Identify the given information:

The fictitious element

has two common isotopes.

Isotope 1:
- Atomic Mass ():
- Abundance ():
Isotope 2:
- Atomic Mass ():
- Abundance (): ?

2. Calculate the abundance of Isotope 2:

Since there are only two common isotopes, their abundances must sum to

3. Convert percentages to fractional abundances:

To use these values in the average atomic mass calculation, we must convert the percentages to decimals by dividing by 100.

Fractional Abundance of Isotope 1 ():
Fractional Abundance of Isotope 2 ():

4. Calculate the average atomic mass ():

The average atomic mass is calculated using the formula:

Substitute the known values:

Calculate the contribution of each isotope:

Sum the contributions:

5. Rounding the final answer:

The masses are given to one decimal place, and the abundance is given to two decimal places (four significant figures in total for the abundance percentage). We should generally round the final answer to a reasonable number of decimal places, often matching the precision of the input masses. Rounding to one decimal place seems appropriate here.

The average atomic mass of

is approximately