Chemical Kinetics - Result Question 49
Passage Based Questions
Passage
Carbon-14 is used to determine the age of organic material. The procedure is based on the formation of ${ }^{14} C$ by neutron capture in the upper atmosphere.
$ { } _7^{14} N+{ } _0 n^{1} \longrightarrow{ } _6^{14} C+{ } _1 p^{1} $
${ }^{14} C$ is absorbed by living organisms during photosynthesis. The ${ }^{14} C$ content remains constant in living organisms once the plant or animal dies, the uptake of carbon dioxide by it ceases and the level of ${ }^{14} C$ in the dead organism falls due to the decay which $^{14}C$ undergoes
$ { } _6^{14} C \longrightarrow{ } _7^{14} N+\beta^{-} $
The half-life period of ${ }^{14} C$ is $5770 yr$.
The decay constant $(\lambda)$ can be calculated by using the following formula $\lambda=\frac{0.693}{t _{1 / 2}}$. The comparison of the $\beta^{-}$activity of the dead matter with that of the carbon still in circulation enables measurement of the period of the isolation of the material from the living cycle. The method however, ceases to be accurate over periods longer than 30,000 yr. The proportion of ${ }^{14} C$ to ${ }^{12} C$ in living matter is $1: 10^{12}$.
49. What is the age of a fossil for meaningful determination of its age?
(a) $6 $ $yr$
(b) $6000 $/yr
(c) $60,000/year
(d) It can be used to calculate age
Show Answer
Answer:
Correct Answer: 49.(b)
Solution:
Fossil whose age is closest to the half-life of $C-14$ ( $5770 $ $yr)$ will yield the most accurate age by $C-14$ dating.
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