NCERT Class XII Chapter <br>Nuclei <br> Questions With Solutions

NCERT Class XII Chapter
Nuclei
Questions With Solutions

Question : 1 of 31

Marks: +1, -0

(a) Two stable isotopes of lithium

\,{}^{6}_{3}\mathrm{Li}

and

\,{}^{7}_{3}\mathrm{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}\mathrm{B}

and

\,{}^{11}_{5}\mathrm{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}\mathrm{B}

and

\,{}^{11}_{5}\mathrm{B}

Solution:

Abundance of

\,{}^{6}_{3}\mathrm{Li}

is 7.5% and abundance of

\,{}^{7}_{3}\mathrm{Li}

is 92.5%.

Hence atomic mass of lithium,

A =

\frac{7.5(6.01512u)+92.5(7.01600u)}{100}

\frac{451134+648.98}{100}

u = 6.941 u

(b) Let abundance of

\,{}^{10}_{5}\mathrm{B}

is x % than abundance of

\,{}^{11}_{5}\mathrm{B}

will be (100 – x)%.

Atomic mass of boron =

\frac{x[10.01294u]+(100-x)[11.00931u]}{100}

⇒ 100 × 10.811 u = 1100.931 u – 0.99637x u

Solving we get, x =

\frac{19.831}{0.99637}

= 19.9 %

So, relative abundance of

\,{}^{10}_{5}\mathrm{B}

isotope = 19.9%

Relative abundance of

\,{}^{11}_{5}\mathrm{B}

isotope = 80.1%

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