Question

A certain radio active substance has a mass of 16 gms at a moment. The mass that disintegrates in its second half life is

• A. 8 gms
• B. 4 gms
• C. 2 gms
• D. 16 gms

Answer 1

Answer: (B)

4 gms

$\begin{array}{c}\text{Original mass}\left(N_0\right)=16g\\ \text{we know that,}\\ \text{Mass of substance left}={\left(\frac{1}{2}\right)}^n\times \text{original mass}\\ n=\text{no. of half life}\\ \therefore N={\left(\frac{1}{2}\right)}^2\times 16\\ =\frac{1}{4}\times 16\\ N=4g\\ \therefore 4g\text{mass left.}\end{array}$