A sample of oxygen contains two isotopes of oxygen with masses 16 u and 18 u respectively. The proportion of these isotopes in the sample is 3 : 1. What will be the average atomic mass of oxygen in this sample?

The average atomic mass of an element is calculated as the weighted average of its isotopes based on their relative abundance. In this sample, oxygen isotopes with masses 16 u and 18 u are present in a 3:1 ratio. This means that 75% (3/4) of the atoms have a mass of 16 u and 25% (1/4) have a mass of 18 u. To find the average atomic mass, we multiply each isotopic mass by its respective fractional abundance and sum the results: (16 × 0.75) + (18 × 0.25). This calculation yields 12 + 4.5, resulting in an average atomic mass of 16.5 u. This method reflects how relative atomic mass is determined for elements with multiple stable isotopes, where the final value is weighted toward the more abundant isotope [1].

Sources

1. [1] https://en.wikipedia.org/wiki/Oxygen_isotope_ratio_cycle