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NCERT Class XII Chapter
Nuclei
Questions With Solutions

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Question : 1
Total: 31
(a) Two stable isotopes of lithium
6
‌
3
Li and
7
‌
3
Li have respective abundances of 7.5% and 92.5%. These isotopes have masses 6.01512 u and 7.01600 u respectively. Find the atomic mass of lithium.
(b) Boron has two stable isotopes,
10
‌
5
B and
11
‌
5
B. Their respective masses are 10.01294 u and 11.00931 u, and the atomic mass of boron is 10.811 u.
Find the abundances of
10
‌
5
B and
11
‌
5
B
Solution:  
Abundance of
6
‌
3
Li is 7.5% and abundance of
7
‌
3
Li is 92.5%.
Hence atomic mass of lithium,
A =
7.5(6.01512u)+92.5(7.01600u)
100
 =
451134+648.98
100
 u = 6.941 u
(b) Let abundance of
10
‌
5
B is x % than abundance of
11
‌
5
B will be (100 – x)%.
Atomic mass of boron =
x[10.01294u]+(100−x)[11.00931u]
100

⇒ 100 × 10.811 u = 1100.931 u – 0.99637x u
Solving we get, x =
19.831
0.99637
 = 19.9 %
So, relative abundance of
10
‌
5
B isotope = 19.9%
Relative abundance of
11
‌
5
B isotope = 80.1%
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