Question

$\begin{array}{ccc}Isotope& RelativeAbudance\left(\%\right)& AtomicMass\left(amu\right)\\ {}^{12}C& 98.892& 12\\ {}^{13}C& 1.108& 13.00335\\ {}^{14}C& 2\times {10}^{-10}& 14.00317\end{array}$

From the data given in the table, calculate the average atomic mass of C.

• A. 13 u
• B. 12.011 u
• C. 12.001 u
• D. 12.0011 u

Answer 1

The correct option is B 12.011 u

Average atomic mass is the weighted average of atomic mass of all the isotopes.

Contribution by each isotope = Relative abundance$\times$Atomic mass

Contribution by
${}_6^{12}C$ = (0.98892)(12)/100 = 11.86704

${}_6^{13}C$ = (0.01108)(13.00335)/100 = 0.14407712

${}_6^{14}C$ = (2 $\times$ ${10}^{-10}$)(14.003170/100 = 28.00634 $\times$ ${10}^{-12}$

$\therefore$ Average atomic mass = sum of all contribution = 12.011 u