Question

An element $X$ has the following isotopic composition: ${}^{200}\text{X}-90\%,{}^{199}\text{X}-8\%,{}^{202}\text{X}-2\%$. The weighted average atomic mass of the naturally occurring element $X$ is closest to:

• A. $201\text{amu}$
• B. $202\text{amu}$
• C. $199\text{amu}$
• D. $200\text{amu}$

Answer 1

The correct option is D $200\text{amu}$

Average atomic weight =
$\frac{(\text{\% abundance}\times \text{atomic weight}{)}_1+\text{(\% abundance}\times \text{atomic weight}{)}_2+(\text{\% abundance}\times \text{atomic weight}{)}_3}{100}$
$=\frac{(90\times 200)+(8\times 199)+(2\times 202)}{100}=199.96\text{amu}\sim 200\text{amu}$