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A radioactive nucleus has a half-life of 20 years, starting with 100,000 particles, how many...

Chemistry
JAMB 2019

A radioactive nucleus has a half-life of 20 years, starting with 100,000 particles, how many particles will be left exactly at the end of 40 years

  • A. 75,000 particles
  • B. 35,000 particles
  • C. 25,000 particles
  • D. 50,000 particles
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Correct Answer: Option C
Explanation

Given:

t\(_{\frac{1}{2}\) = 20 years

After the first 20 years, half of the substance (\(\frac{1}{2} \times 100,000 = 50,000\)) will have decayed. Hence, we have 100,000 - 50,000 = 50,000 particles left.

After the second 20 years (being 40 years in all), half of the remaining substance (\(\frac{1}{2} \times 50,000 = 25,000\)) will have decayed. 

Remaining particles after 40 years = 50,000 - 25,000

= 25,000 particles.

There is an explanation video available below.


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Explanation Video

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Post-UTME Past Questions - Original materials are available here - Download PDF for your school of choice + 1 year SMS alerts
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Post-UTME Past Questions - Original materials are available here - Download PDF for your school of choice + 1 year SMS alerts
WAEC May/June 2024 - Practice for Objective & Theory - From 1988 till date, download app now - 99995
WAEC offline past questions - with all answers and explanations in one app - Download for free
WAEC Past Questions, Objective & Theory, Study 100% offline, Download app now - 24709