Question

An element, $X$ has the following isotopic composition; ${ }^{200} X : 90 \%$ ${ }^{199} X : 8.0 \%$ ${ }^{202} X : 2.0 \%$ The weighted average atomic mass of the naturally-occurring element $X$ is closest to:

• A. 201 amu
• B. 202 amu
• C. 199 amu
• D. 200 amu

Answer 1

Answer: (D)

200 amu

Answer 2

Weight of $^{200}X = 0.90 \times 200 = 180.00 \, u$
Weight of $^{199}X = 0.08 \times 199 = 15.92 \, u$
Weight of $^{202}X = 0.02 \times 202 = 4.04 \, u$
Total weight $= 199.96 \approx 200 \, u$