Question

A certain sample of element Z contains 60% of ${}^{69}Z$ and 40% of ${}^{71}Z$. What is the average atomic mass of element Z in this sample?

• A. 69.2
• B. 69.8
• C. 70.0
• D. 70.2

Answer 1

The correct option is B

69.8

Let the mass of the first isotope be $m_1$,
the percentage of the first isotope be $p_1$,
the mass of the second isotope be $m_2$,
the percentage of the second isotope be $p_2$

Then, the average atomic mass of the element

$=\frac{m_1\times p_1+m_2\times p_2}{100}$

$=\frac{60\times 69+71\times 40}{100}=69.8$