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Nucleus: Common Terms

Naturally occ...

Question

Naturally occurring argon (Ar) consists of three isotopes the atoms of which occur in the abundances as given in the table The atomic weight of argon is:
Isotopes Abundance
$^{36} A r$ 0.34%
$^{38} A r$ 0.07%
$^{40} A r$ 99.59%

A

38.45 u

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B

37.20 u

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C

39.98 u

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D

40.98 u

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Solution

The correct option is D 39.98 u
For different isotopes, atomic wt is the avg wt. of all isotopes so
atomic weight $= (99.59 \times 40 + 0.07 \times 38 + 0.34 \times 36) / 100 = 39.98 u$

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