An element has two naturally occurring isompes widx atomic masses of 10.01 amu and 11.01 amu. The relative abundances of these elements are 19.8% and 80.2%, respectively. What is the average atomic mass of the element?

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Answer 1 · 2015-11-29T14:51:18

#10.812u#

Now I hope you have been exposed to the concept of weighted average;
If not here is a video

Now lets get our data;

Lets call our element x; Let its existence and abundance be divided in 100 parts

so

#10.01 #amu is present in 19.8parts

Total atomic mass in this isotope is;

#10.01 *19.8 = 198.198 # mass units

#11.01 #amu is present in 80.2parts

Total atomic mass in this isotope is;

#11.01 *80.2= 883.002 # mass units

Now total number of mass units #= 883.002 +198.198= 1081.2#

But now the total number of parts# = 100#

So all that's left is the weighted average;

#= 1081.2/100= 10.812 u#