Question

In a radioactive sample $$^{40}_{19}K$$ nuclei either decay into stable $${^{40}_{20}Ca}$$ nuclei with decay constant $$4.5\times 10^{-10}$$ per year or into stable $$^{40}_{18}Ar$$ nuclei with decay constant $$0.5\times 10^{-10}$$ per year. Given that in this sample all the stable $$^{40}_{20}Ca$$ and $$^{40}_{18}Ar$$ nuclei are produced by the $$^{40}_{19}K$$ nuclei only. In time $$t\times 10^{9}$$ years, if the ratio of the sum of stable $$^{40}_{20}Ca$$ and $$^{40}_{18}Ar$$ nuclei to the radioactive $$^{40}_{19}K$$ nuclei is $$99$$, the value of t will be? [Given: In $$10=2.3$$]

Answer 1

Correct option is A. $$9.2\times10^9$$

$$t=0$$

$$\dfrac{dN}{dt}=2\left(\lambda_1+\lambda_2\right)\times N$$
$$log_e\left(\dfrac{N}{N_0}\right)=-(\lambda_1+\lambda_2)t$$
$$2.3\times log_{10}\left(\dfrac{N_0}{N_0/100}\right)=5\times 10^{-10}t$$
$$\dfrac{2.303\times 2}{5\times 10^{-10}}=t$$
$$2.303\times 0.4\times 10^{10}=t$$
$$t=9.2\times 10^9$$ year.