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Test Index
NCERT Class XII Chapter
Nuclei
Questions With Solutions
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© examsnet.com
Question : 1
Total: 31
(a) Two stable isotopes of lithium
6
3
L
i
and
7
3
L
i
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
Validate
Solution:
Abundance of
6
3
L
i
is 7.5% and abundance of
7
3
L
i
is 92.5%.
Hence atomic mass of lithium,
A =
7.5
(
6.01512
u
)
+
92.5
(
7.01600
u
)
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
[
10.01294
u
]
+
(
100
−
x
)
[
11.00931
u
]
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%
© examsnet.com
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