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The half-life of a radioactive substance is 15 hours. If at some instance, the sample...

Physics
WAEC 2022

The half-life of a radioactive substance is 15 hours. If at some instance, the sample has a mass of 512 g, calculate the time it will take \(\frac{7}{8}\) of the sample to decay

  • A. 15 hours
  • B. 30 hours
  • C. 45 hours
  • D. 60 hours
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Correct Answer: Option C
Explanation

Initial Mass: 512 g, Mass Decayed: \(\frac{7}{8}\) of 512 = 448g, Remaining mass = 512 - 448 = 64g

Using, N(t) = No(\(\frac{1}{2}\))\(^{\frac{T}{T1/2}}\)

\(\frac{N(t)}{No}\) = (\(\frac{1}{2}\))\(^{\frac{T}{T1/2}}\)

\(\frac{64}{512}\) = \(\frac{1}{8}\) =  (\(\frac{1}{2}\))\(^{\frac{T}{15}}\)

\(\frac{1}{8}\) = (\(\frac{1}{2}\))\(^3\) =  (\(\frac{1}{2}\))\(^{\frac{T}{15}}\)

equating coefficient

3 = \(\frac{T}{15}\)

T = 3 x 15 = 45hours.


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