Question

C-12 and c-13 are naturally-occurring isotopes of the element carbon. C-12 occurs 98.89% of the time and c-13 occurs 1.108% of the time. What calculation should be used to determine the atomic mass of this element?

Answer 1

Answer:

$\boxed{\text{12.011 u}}$

Explanation:

You calculate the weighted average of the atomic masses.

That is, you multiply the atomic mass of each isotope by a number representing its relative importance (e.g.., its percent of the total).

Set up a table for easy calculation:

$\begin{array}{ccrcr}\textbf{Atom} & \textbf{Mass/u} &\textbf{Percent} & \textbf{Calculation}& \textbf{Result/u}\\^{12}\text{C}& 12.000 & 98.892 & 12.000 \times 0.988 92 & 11.867\\^{13}\text{C}& 13.003 & 1.108 & 13.003 \times 0.011 08 & 0.144 \\& & & \text{TOTAL} = &\textbf{12.011}\\\end{array}\\\\\text{The average atomic mass of C is }\boxed{\textbf{12.011 u}}$