Question

Using the isotopic masses 34.98 u (75.4% abundance) and 36.98 u (24.6% abundance), the average atomic mass of chlorine is approximately:

Options

• \[71.96~\mathrm{u}\]

• \[34.98~\mathrm{u}\]

• \[36.98~\mathrm{u}\]

• \[35.46~\mathrm{u}\]

Answer 1

The average mass is the weighted mean: \[\frac{(34.98 \times 75.4) + (36.98 \times 24.6)}{100} = \frac{2637.5 + 909.7}{100} \approx 35.46~\mathrm{u}\]. This matches the measured atomic mass of chlorine and explains why it is not a whole number.