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A sample of oxygen contains only O¹⁶ and O¹⁸ isotopes with percentage abundances of 90% and 10% respectively. Which of the following statements are correct?

1. The average atomic mass of the sample is 16.2.
2. The average number of protons per atom is 8.
3. The average number of neutrons per atom is 8.2.
4. The molecular mass of oxygen gas formed from this sample can range from 32 to 36.

Correct answer: The average atomic mass of the sample is 16.2.

Solution

Average atomic mass = 16.2 (correct). All oxygen atoms have 8 protons (correct). Average neutrons = 0.9*8 + 0.1*10 = 8.2 (correct). O2 molecule masses can be 32, 34, or 36 — not just 32 to 36 skipping 34, so option D requires the word 'can vary' (which is true). All four options are correct.

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