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JAMB Chemistry 2011 Paper

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Question : 5

Total: 47

The relative atomic mass of a naturally occurring lithium consisting of 90%

{}^7_3

{}^6_3

10% Li is

6.9
7.1
6.2
6.8

Solution:

Concept:

The relative atomic mass is the weighted average of the masses of all naturally occurring isotopes, based on their relative abundances.

Explanation:

Lithium consists of

90\%

{}^7_3\mathrm{Li}

and

10\%

{}^6_3\mathrm{Li}

.

Use the formula:

R.A.M = (m_1 \times \alpha_1) + (m_2 \times \alpha_2)

Substitute the values:

R.A.M = 7 \times \frac{90}{100} + 6 \times \frac{10}{100}

Calculate:

R.A.M = 7(0.9) + 6(0.1)

So:

R.A.M = 6.3 + 0.6 = 6.9

Answer:

Relative atomic mass

= 6.9

(Option A).

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