Question

A sample of lead is found to contain three isotopes: 23.5% of the sample has a mass of 204 amu, 24.1% has a mass of 207 amu, and 52.4% has a mass of 208 amu. According to your calculations, what is the average atomic mass of lead?

Answer 1

The average atomic mass of lead is 206.819 amu

Let the 1st isotope be A

Let the 2nd isotope be B

Let the 3rd isotope be C

From the question given above, the following data were obtained:

For Isotope A

Abundance of A (A%) = 23.5%

Mass of A = 204 amu

For Isotope B

Abundance of B (B%) = 24.1%

Mass of B = 207 amu

For Isotope C

Abundance of C (C%) = 52.4%

Mass of C = 208 amu

Average atomic mass of lead =?

The average atomic mass of lead can be obtained as follow:

Atomic mass = [(Mass of A × A%)/100] + [(Mass of B × B%)/100] + [(Mass of C × C%)/100]

= [(204 × 23.5)/100] + [(207 × 24.1)/100] +  [(208 × 52.4)/100]

= 47.94 + 49.887 + 108.992

= 206.819 amu

Therefore, the average atomic mass of lead is 206.819 amu

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