Question

A newly discovered element, Y, has two naturally occurring isotopes. 87.8 percent of the sample is an isotope with a mass of 267.8 u, and 12.2 percent of the sample is an isotope with a mass of 269.9 u. What is the weighted average atomic mass for this element?

Answer 1

Answer : The average atomic mass of an element is, 268.6 amu.

Solution : Given,

Mass of isotope 1 = 267.8 amu

% abundance of isotope 1 = 87.8% = 0.878

Mass of isotope 2 = 269.9 amu

% abundance of isotope 2 = 12.2% = 0.122

Formula used for average atomic mass of an element :

$\text{ Average atomic mass of an element}=\sum(\text{atomic mass of an isotopes}\times {{\text { fractional abundance}})$

$\text{ Average atomic mass of an element} Y=\sum[(267.8\times 0.878)+(269.9\times 0.122)$

$\text{ Average atomic mass of an element}=268.06amu$

Therefore, the average atomic mass of an element is, 268.6 amu.

Answer 2

(0.878)(267.8) + (0.122)(269.9)=

268 u (three sig fig) 


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