Calculating Half-Life: 0.48g to 0.003g

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SUMMARY

The discussion focuses on calculating the initial amount of a radioactive substance based on its half-life of 5 hours. A sample containing 0.48g was tested, and the user initially calculated the decay over time incorrectly by moving forward instead of backward. The correct approach involves determining how much of the substance existed 20 hours prior, which corresponds to four half-lives, resulting in an initial amount of 0.48g increasing to 0.003g after decay.

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  • Understanding of radioactive decay and half-life concepts
  • Basic algebra for calculating exponential decay
  • Familiarity with the formula for half-life: N = N0 * (1/2)^(t/T)
  • Knowledge of time intervals in relation to half-lives
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  • Learn about radioactive decay series and their applications
  • Explore the implications of half-life in nuclear physics
  • Review examples of half-life problems in chemistry textbooks
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Students in physics or chemistry, educators teaching radioactive decay, and anyone interested in understanding half-life calculations in practical scenarios.

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Homework Statement



The Half-life of a radioactive substance is 5 hours. A sample is tested and found to contain 0.48g of the substance.
How much of the substance was present in the sample 20 hours before the sample was tested?

Homework Equations





The Attempt at a Solution


i did like this...

20/5 = 4 half life

0.48---->0.24--->0.12--->0.06--->0.003g amount of substance was present.

am i right?
 
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Zoheb Imran said:
How much of the substance was present in the sample 20 hours before the sample was tested?

Right idea, but you want the other direction in time...
 
why the other direction? can you please explain? that's where i am having problem...in calculating...
 
Zoheb Imran said:
why the other direction? can you please explain? that's where i am having problem...in calculating...

You are calculating forward in time when you used the half-life. So you are figuring out how much of the radioactive material is left after it was sampled. The question asks you to figure out how much of the material was present before the sample was made...
 
thanks!
 

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