Question

isotopes and average atomic mass

1. which of the following are not an isotope of carbon?

the average atomic mass formula is: ((mass#1xpercent abundance) + (mass#2xpercent abundance) + ....)/100

1. calculate the average atomic mass of the following: show all work!!!

isotope mass (amu) percent abundance
magnesium - 24 23.985 78.70%
magnesium - 25 24.986 10.03%
magnesium - 26 25.983 11.17%

1. calculate the average atomic mass of the following: show all work!!!

isotope mass (amu) percent abundance
lithium - 6 6.015 7.50%
lithium - 7 7.016 92.50%

Explanation:

Step1: Recall isotope definition

Isotopes of an element have the same number of protons but different numbers of neutrons. Carbon has 6 protons. Without seeing the details of the diagrams (A, B, C), we can't answer the first part.

Step2: Calculate average atomic mass of magnesium

Use the formula $\frac{(mass\#1\times percent\ abundance)+(mass\#2\times percent\ abundance)+(mass\#3\times percent\ abundance)}{100}$.
\[

$$\begin{align*} &=\frac{(23.985\times78.70)+(24.986\times10.03)+(25.983\times11.17)}{100}\\ &=\frac{(23.985\times0.787)+(24.986\times0.1003)+(25.983\times0.1117)}{1}\\ &=\frac{18.876295 + 2.5060958+2.9023011}{1}\\ &=\frac{24.2846919}{1}\\ &\approx 24.31 \end{align*}$$

\]

Step3: Calculate average atomic mass of lithium

\[

$$\begin{align*} &=\frac{(6.015\times7.50)+(7.016\times92.50)}{100}\\ &=\frac{(6.015\times0.075)+(7.016\times0.925)}{1}\\ &=\frac{0.451125+6.4898}{1}\\ &=\frac{6.940925}{1}\\ &\approx 6.94 \end{align*}$$

\]

Answer:

1. Can't answer without diagram details
2. Approximately 24.31 amu
3. Approximately 6.94 amu