Question

An isotope of potassium ${}_{19}^{40}K$ has a half life of 1.4 × 10⁹ year and decays to Argon ${}_{18}^{40}Ar$which is stable. A sample of rock taken from the moon contains both potassium and argon in the ratio 1/7. Age of the rock is –

• A. 2.8 × 10⁹ year
• B. 3.5 × 10⁹ year
• C. 4.2 × 10⁹ year
• D. 8.2 × 10¹⁰ year

Answer 1

Answer: (C)

4.2 × 10⁹ year

Answer 2

N =

= $\frac { \mathrm { N } _ { 0 } } { \mathrm {~N} }$

= $\frac { 8 } { 1 }$

t = $3 \mathrm {~T} _ { 1 / 2 }$ = 4.2 × 10⁹ year