Question

Calculate the mass of ${}^{140}La$ in a sample whose activity is $3.7\times {10}^{10}Bq$ ( 1 becquerel, Bq = 1 disintegration pr second) given that its $t_{1/2}$ is 40 hr.

• A. $1.8\times {10}^{-6}g$
• B. $2.8\times {10}^{-6}g$
• C. $5.8\times {10}^{-6}g$
• D. None of these

Answer 1

The correct option is A $1.8\times {10}^{-6}g$

$-\frac{dN}{dt}=\lambda N$
$3.7\times {10}^{10}=\frac{0.693\times w\times 6.02\times {10}^{28}}{40\times 60\times 60\times 140}$
$\therefore w=1.8\times {10}^{-6}g$