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
${}^{36}Ar$	0.34%
${}^{38}Ar$	0.07%
${}^{40}Ar$	99.59%

• A. 38.45 u
• B. 37.20 u
• C. 39.98 u
• D. 40.98 u

Answer 1

Answer: (C)

39.98 u

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