Tritium has a half life of 12.5 years against beta decay. What fraction of a sample of pure tritium disintegrates in 25 years?

$\frac{3}{4}$

The correct answer is Option (3) → $\frac{3}{4}$

Radioactive decay,

$N(t)=N_0\left(\frac{1}{2}\right)^{\frac{t}{T_{1/2}}}$

$N(t)$ = Amount of tritium remaining after time t

$N_0$ = Initial amount of tritium

$T_{1/2}$ = Half life of substance

$N(25)=N_0\left(\frac{1}{2}\right)^{\frac{25}{12.5}}$

$N(25)=N_0×\frac{1}{4}$

∴ Fraction disintegrated = $1-\frac{N(25)}{N_0}=1-\frac{1}{4}=\frac{3}{4}$