The half life of a first order reaction is 2000 years. If the concentration after 8000 years is 0.02M, then the inital concentration was.

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0.16M

0.08M

0.04M

0.32M

answer is B.

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Detailed Solution

For first order reaction, concentration present after ${nt}_{\frac{1}{2}} = \frac{a}{2^{n}}$
a = Initial concentration
n = no . of $t_{1 / 2}$
$\begin{array}{l} n = \frac{8000}{2000} = 4 \\ \frac{a}{2^{4}} = 0.02 \\ a = 0.32 M \end{array}$

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