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A sample of radioactive element has a mass of 10 g at an instant t=0. The approximate mass of this element in the sample after two mean lives is

Asked in CBSE AIPMT 2003 · Decay law and half-life

• (1) 3.70 g
• (2) 6.30 g
• (3) 1.35 g
• (4) 2.50 g

Step-by-step solution

Why the other options are wrong

• (1) comes from is 10e⁻¹, the mass left after one mean life rather than two.
• (2) comes from is 10(1-e⁻¹), the mass that has decayed in one mean life instead of the mass remaining.
• (4) comes from treats two mean lives as two half-lives, which would leave a quarter of 10 g.

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