
To:
Find the radioactivity of the Radium in the watch.

Answer to Problem 21.58QA
Solution:
Explanation of Solution
1) Concept:
The radioactivity is related to the number of atoms decaying by the relation
2) Formulae:
i)
ii)
iii)
iv)
3) Given:
i) Half-life of Radium-226
ii) Initial quantity of Radium-226 used
iii) Initial time
iv) Final time
4) Calculations:
Half-life of the Radium-226 is given in years, which we have to convert it into seconds because the radioactivity is measured in seconds. Therefore, the half-life in seconds is
The decay rate constant for the decay of Radium-226 is:
The number of atoms in
The time interval is:
We have to convert this time in seconds, which is done as:
The number of atoms remaining after decay can be found out by the following relation:
Now, we know the atoms remaining, we can find out the radioactivity remaining in the watch.
Thus, the radioactivity of the watch today, is
Now, we know that
Therefore, the radioactivity in Ci is:
Thus, the radioactivity of the watch today is
Conclusion:
The radioactivity left after a certain time period can be found out if we have the initial quantity and the half-life of the radioactive element.
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Chapter 21 Solutions
Chemistry: An Atoms-Focused Approach
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