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Class 12
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a two stable isotopes of lithium 6li3 and7li3 ha...

Question:
(a) Two stable isotopes of lithium 6Li3 and7Li3 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, ¹⁰B₅ and ¹¹B₅ . 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 ¹⁰B₅ and ¹¹B₅.

Answer:

(a) Mass of ⁶Li₃, lithium isotope, m 1 = 6.01512 u

Mass of ⁷Li₃, lithium isotope, m = 7.01600 u

Abundance of ⁶Li₃ , η₁ = 7.5%

Abundance of ⁷Li₃ , η₂ = 92.5%

The atomic mass of lithium atom is given as:

( b ) Mass of boron isotope ¹⁰B₅ , m = 10.01294 u

Mass of boron isotope ¹¹B₅ , m = 11.00931 u

Abundance of ¹⁰B₅, η₁ = x %

Abundance of ¹¹B₅, η₂ = (100 − x ) %

Atomic mass of boron, m = 10.811 u The atomic mass of boron atom is given as:

And 100 − x = 80.11%

Hence, the abundance of is ¹⁰B₅ 19.89% and that of ¹¹B₅ is 80.11%.

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