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(half-life = 15 hrs.) is known to contain some radioactive impurity (half-life = 3 hrs.) in a sample. This sample has an initial activity of 1000 counts per minute, and after 30 hrs it shows an activity of 200 counts per minute. What percent of the initial activity was due to the impurity ?
  • a)
    10
  • b)
    40
  • c)
    5
  • d)
    20
Correct answer is option 'D'. Can you explain this answer?
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(half-life = 15 hrs.) is known to contain some radioactive impurity (h...
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(half-life = 15 hrs.) is known to contain some radioactive impurity (h...
Let the activity due to impurity be 'a' cpm.
∴ due to Na it is (1000 – a) cpm.
After 30 hrs 'a' would be reduced to   a cpm and (1000 – a) would be reduced to  cpm
∴ total activity after 30 hrs would be
= 200 (given)
solving we get 
 
∴  ⇒ a = 200
Hence 20% activity was due to impurity.
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(half-life = 15 hrs.) is known to contain some radioactive impurity (half-life = 3 hrs.) in a sample. This sample has an initial activity of 1000 counts per minute, and after 30 hrs it shows an activity of 200 counts per minute. What percent of the initial activity was due to the impurity ?a)10b)40c)5d)20Correct answer is option 'D'. Can you explain this answer?
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