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?
- A.17.5 u
- B.17 u
- C.16 u
- D.16.5 u
Correct answer
Explanation
The correct answer is D, 16.5 u. Three parts of oxygen-16 and one part of oxygen-18 give an average of (3 × 16 + 1 × 18) ÷ 4 = 66 ÷ 4 = 16.5 u.
Isotopes are atoms of the same element with the same atomic number but different mass numbers, because they have different numbers of neutrons. The atomic mass of an element is the weighted average of the masses of its isotopes, each multiplied by its share in the sample. This is why many elements have fractional atomic masses; chlorine, a mix of chlorine-35 and chlorine-37 in about 3 : 1, has an atomic mass of about 35.5 u.
A is wrong: 17.5 u would be the result if the proportion were reversed, 1 : 3.
B is wrong: 17 u is the simple average of 16 and 18, which ignores the 3 : 1 proportion.
C is wrong: 16 u would mean the sample had no oxygen-18 at all.
Exam tip: average atomic mass = sum of (isotope mass × its fraction); chlorine is 35.5 u from Cl-35 and Cl-37 in 3 : 1.
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