# Half-life

Half –life

Half-time is the period of time taken for the amount of substance undergoing decay to decrease by half. Hife-life is used to describe entities, such as radioactive atoms.

Some of the important questions and answers of Half-life are listed below:-

Worksheet Questions:-

1. The value of $t_{1/2} \text{and} \lambda$ depend upon:

(a)    Mass of substance

(b)   External factors like temperature, pressure

(c)    Physical and chemical changes

(d)   None of these

2. If $t_{1/2}$ is 1580 years the value of average life and disintegration constant are respectively?

(a) $2275.2 \text{years}, 4.38 \times 10^{-4} \text{year}^{-1}$

(b) $2376 \text{years}, 4.36 \times 10^{-2} \text{year}^{-1}$

(c) $2478 \text{years}, 4.32 \times 10^{-5} \text{year}^{-1}$

(d) $2480 \text{years}, 4.8 \times 10^{-3} \text{year}^{-1}$

3. If $t_{1/2}$ of a radio active substance of 10 kg is 100 years what will it be for 10 gm of this substance at very high pressure?

(a)    200 yrs.

(b)   100 yrs.

(c)    50 yrs.

(d)   400 yrs.

4. The half-life of a radio active] compound is 50 years. What % of it will de compose after 200 yrs.

(a)    75 %

(b)   82. 5%

(c)    93.75%

(d)   96.25%

5. The isotopic mass of $^{238}_{92}U$ is 238.125 a.m.u. its packing fraction is:

(a)    5.25

(b)   6.25

(c)    12.5

(d)   Zero

6. The half-life of radioactive K-40 is 5.27 years, and $\lambda$ is $2.5 \times 10^{-7} min^{-1}$. The decay activity of 2 gm of the sample is nearly?

(a) $2.5 \times 10^{5} dpm$

(b) $7.5 \times 10^1{5} dpm$

(c) $7.5 \times 10^{12}dpm$

(d) $5 \times 10^{15} dpm$