Chemical Kinetics - Result Question 48
Passage Based Questions
Passage
Carbon-14 is used to determine the age of organic material. The procedure is based on the formation of ${ }^{14} Cby$ 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 is constant in living organism once the plant or animal dies, the uptake of carbon dioxide by it ceases and the level of ${ }^{14} C$ in the dead being, falls due to the decay which $C$ - 14 underoges
$ { } _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 $1: 10^{12}$.
48. Which of the following option is correct?
(a) In living organisms, circulation of ${ }^{14} C$ from atmosphere is high so the carbon content is constant in organism
(b) Carbon dating can be used to find out the age of earth crust and rocks
(c) Radioactive absorption due to cosmic radiation is equal to the rate of radioactive decay, hence the carbons content remains constant in living organisms
(d) Carbon dating cannot be used to determine concentration of ${ }^{14} C$ in dead beings
Show Answer
Answer:
Correct Answer: 48. (c)
Solution:
- Living plants maintain an equilibrium between the absorption of $C^{14}$ (produced due to cosmic radiation) and the rate of decay of $C^{14}$ present inside the plant. This gives a constant amount of $C^{14}$ per gram of carbon in a living plant.
BETA
