# If ratio of molecular masses of two radioactive substances is $\;\large\frac{1}{2}\;$ and ratio of decay constant is $\;\large\frac{5}{3}\;$ . Then ratio of their initial activity per mole will be

$(a)\;\large\frac{4}{3}$$\qquad($$b)\;\large\frac{5}{3}\qquad($$c)\;$$3\qquad(d)\;\large\frac{2}{3}$

Answer : (b) $\;\large\frac{5}{3}$
Explanation :
Activity $\;R=\lambda N$
No .of nuclei (N) per mole are equal for both substances
$R \propto \lambda$
or $\;\large\frac{R_{1}}{R_{2}}=\large\frac{\lambda_{1}}{\lambda_{2}}=\large\frac{5}{3}\;.$