Substance A has atomic mass number 16 and half life of 1 day. Another substance B has atomic mass number 32 and half life of 1/2 day. If both A and B simultaneously start undergoing radioactivity at the same time with initial mass 320 g each, how many total atoms of A and B combined would be left after 2 days?
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Question

Substance A has atomic mass number 16 and half life of 1 day. Another substance B has atomic mass number 32 and half life of 1/2 day. If both A and B simultaneously start undergoing radioactivity at the same time with initial mass 320 g each, how many total atoms of A and B combined would be left after 2 days?

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G1 Physics · Accepted Answer

For A: initial amount = 320/16 = 20 mol. After 2 days = two half-lives, so remaining = 20/2² = 5 mol. For B: initial amount = 320/32 = 10 mol. Since half-life = 0.5 day, 2 days = four half-lives, so remaining = 10/2⁴ = 0.625 mol. Total = 5.625 mol. Number of atoms = 5.625 × 6.023 × 10²³ ≈ 3.38 × 10²⁴.