Question

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

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

Answer 1

The correct option is B

$69.8$

1. The average of the mass numbers of the different isotopes is known as the relative atomic mass.
2. Relative atomic mass takes into account the percentage abundance of the isotopes.
3. Atoms of the same element having different number of neutrons are known as isotopes.

Here, the given isotopes of Z are: ${}_{}{}^{69}Z$ (60%) and ${}_{}{}^{71}Z$(40% ).

Therefore, relative atomic mass = $\frac{60\times 69+40\times 71}{100}=\mathit{}\mathit{69}\mathit{.}\mathit{8}$

Hence, the relative atomic mass of Z is 69.8.