
Concept explainers
(a)
The decay constant for 14C.
(a)

Answer to Problem 41P
The decay constant for 14C is 3.83×10−12 s−1.
Explanation of Solution
The activity of 14C in a living sample is 0.25 Bq/g of carbon. The half-life of 14C is 5730 yr
Write the formula for half-life
T1/2=τln2 (I)
Here, T1/2 is the half-life and τ is the time constant.
Write the formula for the decay constant
λ=1τ (II)
Here, λ is the decay constant
Conclusion:
Substitute equation (I) in (II) and rewrite to find the λ
λ=ln2T1/2
Substitute 5730 yr for T1/2 in the above relation to find the value of λ. [Note: 1 yr=3.156×107 s
λ=ln25730 yr×3.156×107 s/yrλ=3.83×10−12 s−1
Thus, the decay constant for 14C is 3.83×10−12 s−1.
(b)
Calculate the number of 14C atoms in 1.00 g of carbon.
(b)

Answer to Problem 41P
The number of 14C atoms in 1.00 g of carbon is 6.5×1010 atoms.
Explanation of Solution
One mole of carbon atom has 12.011 g and the relative abundance of 14C is 1.3×10−12. The
The number of 14C atoms in 1.00 g of carbon is the total number of nuclei in 1.00 g of carbon times the relative abundance of 14C.
N=massmass per mol×NA×relative abundance
Substitute 6.022×1023 atoms/mol for NA, 1.3×10−12 for the relative abundance, 1.00 g for the mass and 12.011 g/mol for mass per mol.
N=1.00 g12.011 g/mol×6.022×1023 atoms/mol×1.3×10−12N=6.5×1010 atoms
Thus, the number of 14C atoms in 1.00 g of carbon is 6.5×1010 atoms.
(c)
The activity of 14C per gram of carbon in a living sample.
(c)

Answer to Problem 41P
The activity of 14C per gram of carbon in a living sample is 0.25 Bq/g.
Explanation of Solution
Write the formula for the activity
R=λN (III)
Here,λ is the decay constant and N is the number of nuclei after time t.
Conclusion:
Substitute 3.83×10−12 s−1 for λ and 6.5×1010 atoms for N in equation (III) to find the value of R
R=3.83×10−12 s−1×6.5×1010 atomsR=0.25 Bq
Divide by 1.00 g on both sides to find the value of activity per gram
R1.00 g=0.25 Bq1.00 gR1.00 g=0.25 Bq/g
Thus, the activity of 14C per gram of carbon in a living sample is 0.25 Bq/g.
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