Question

If $N_0$ is the original mass of the substance of half-life period $t_{1/2}=5$ years, then the amount of substance left after 15 years is

• A. $\frac{N_0}{8}$
• B. $\frac{N_0}{16}$
• C. $\frac{N_0}{2}$
• D. $\frac{N_0}{4}$

Answer 1

The correct option is A $\frac{N_0}{8}$

Number of half lives $n=\frac{15}{5}=3$
We know that $N=N_0{\left(\frac{1}{2}\right)}^n=N_0{\left(\frac{1}{2}\right)}^3=\frac{N_0}{8}$