International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

If a sample of carbon contains 98.90% carbon-12 and 1.10% carbon-13, what is the relative atomic mass of carbon?

1. 11.99

2. 12.00

3. 12.01

4. 12.02

The correct answer is: 12.01

The relative atomic mass of an element can be calculated by taking into account the percentage abundance of its isotopes and their respective atomic masses. In the case of carbon, it primarily exists in two isotopes: carbon-12, which has an atomic mass of 12, and carbon-13, which has an atomic mass of approximately 13. To calculate the relative atomic mass of the carbon sample provided: 1. Multiply the percentage abundance of each isotope by its atomic mass. - For carbon-12: 98.90% (or 0.9890 when converted to decimal) * 12 = 11.868 - For carbon-13: 1.10% (or 0.0110 when converted to decimal) * 13 = 0.143 2. Add these two results together to find the weighted average: - 11.868 + 0.143 = 12.011 When rounded to two decimal places, this gives a relative atomic mass of approximately 12.01. This value is consistent with the known relative atomic mass of carbon in the periodic table, which is often quoted as 12.011, acknowledging the small contributions from heavier isotopes like carbon-13