Question

An element has the following natural abundances and isotopic masses: 90.92% abundance with $19.99\text{amu}$, 0.26% abundance with $20.99\text{amu}$ and 8.82% abundance with $21.99\text{amu}$. Calculate the average atomic mass of this element.

• A. $80.52\text{amu}$
• B. $25.45\text{amu}$
• C. $15.89\text{amu}$
• D. $20.16\text{amu}$

Answer 1

The correct option is D $20.16\text{amu}$

Average atomic mass is given by:
$\frac{(\text{atomic mass}\times \text{\%abundance}{)}_1+(\text{atomic mass}\times \text{\%abundance}{)}_2+(\text{atomic mass}\times \text{\%abundance}{)}_3}{100}$
$=\frac{\left(\right(19.99\times 90.92)+(20.99\times 0.26)+(21.99\times 8.82\left)\right)}{100}=20.16\text{amu}$