University:
Campbell University
Course:
PHYS 252 | Fundamentals of Physics II
Academic year:

2019
Views:

408

Pages:

2
Author:

seminginzms

KEY Half-Life 1.) What is the ratio of 𝑐𝑎𝑟𝑏𝑜𝑛 14 to 𝑁𝑖𝑡𝑟𝑜𝑔𝑒𝑛 14 after two half-lives have occurred? Answer – After one half-life 50% will be left of the parent and 50% will have been made of the daughter atoms. After the second half-life, only 25% will be left of the parent and so 75% will have been made of the daughter atoms. So, the ratio is 25: 75 for parent to daughter atoms or 1:3 if simplified. 2.) 𝑆𝑜𝑑𝑖𝑢𝑚 24 decays to produce 𝑚𝑎𝑔𝑛𝑒𝑠𝑖𝑢𝑚 24. A laboratory sample contains 400 𝑎𝑡𝑜𝑚𝑠 of 𝑠𝑜𝑑𝑖𝑢𝑚 24. Use the decay curve below to answer the following questions: b.) a.) d.) c.) a.) How many 𝑠𝑜𝑑𝑖𝑢𝑚 24 atoms will be left after 30 ℎ𝑟𝑠? 100 𝑎𝑡𝑜𝑚𝑠 b.) How many magnesium-24 atoms will be present after 53 ℎ𝑟𝑠? 30 𝑎𝑡𝑜𝑚𝑠 of sodium-24 left would mean that there is now 400– 30 c.) What is the half-life of 𝑠𝑜𝑑𝑖𝑢𝑚 370 𝑎𝑡𝑜𝑚𝑠 of magnesium-24 24? About 14.5 ℎ𝑜𝑢𝑟𝑠 d.) How many 𝑠𝑜𝑑𝑖𝑢𝑚 24 atoms will be present after 75 ℎ𝑟𝑠? approximately 11 atoms 1 KEY 3.) A radioactive isotope has a mass of 120 𝑔. If the isotope had a half-life of 20 𝑠, what would be the mass of the isotope after 2.0 𝑚𝑖𝑛? If the half-life is 20 𝑠 then there will be 6 half-lives to reach 2 𝑚𝑖𝑛. 6 0.015625% 𝑙𝑒𝑓𝑡 0.015625 120 1.875 𝑔 4.) A granite rock is thought to be about 2 𝑏𝑖𝑙𝑙𝑖𝑜𝑛 years old. Why can’t we use 𝑐𝑎𝑟𝑏𝑜𝑛 14 dating to find out if this is correct? Carbon-14 dating will not work for dating items that were never alive to build up carbon. As well after a billion years there will be too small amount of carbon-14 left to accurately weigh to get a proper date. 5.) An organic sample is 28650 𝑦𝑒𝑎𝑟𝑠 old. What percentage of the original 𝑐𝑎𝑟𝑏𝑜𝑛 14 is still present in the sample? The half-life of carbon-14 is 5730 𝑦𝑒𝑎𝑟𝑠. 5. There has been 5 half-lives. There is left or 0.03125. This amount as a percent is 3.125%. 2

What Percentage of a Radioactive Species Would be Found as Daughter Material After Five Half-lives

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