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?
17.5 u
17 u
16 u
16.5 u
Answer is Right!
Answer is Wrong!
This question was previously asked in
UPSC NDA-2 – 2018
– The sample contains isotopes with masses 16 u and 18 u in a proportion of 3:1.
– This means out of 4 parts, 3 parts have mass 16 u and 1 part has mass 18 u.
– Relative abundance of 16u isotope = 3 / (3+1) = 3/4 = 0.75
– Relative abundance of 18u isotope = 1 / (3+1) = 1/4 = 0.25
– Average atomic mass = (Mass₁ × Abundance₁) + (Mass₂ × Abundance₂)
– Average atomic mass = (16 u × 0.75) + (18 u × 0.25)
– Average atomic mass = 12 u + 4.5 u = 16.5 u.