The half life of the isotope ${}_{11}{}^{24}\mathrm{Na}$ is 15 hrs. How much time does it take for $\frac{7}{8}\mathrm{th}$ of a sample of this isotope to decay

(1) 75 hrs               

(2) 65 hrs

(3) 55 hrs               

(4) 45 hrs

If the half life of a radioactive sample is 10 hours,
its mean life is 

(1) 14.4 hours               

(2) 7.2 hours

(3) 20 hours                 

(4) 6.93 hours

If 20 gm of a radioactive substance due to radioactive decay reduces to 10 gm in 4 minutes, then in what time 80 gm of the same substance will reduce to 10 gm 

(1) In 8 minutes             

(2) In 12 minutes

(3) In 16 minutes             

(4) In 20 minutes

16 gm sample of a radioactive element is taken from Bombay to Delhi in 2 hour and it was found that 1 gm of the element remained (undisintegrated). Half life of the element is 

(1) 2 hour         

(2) 1 hour

(3) $\frac{1}{2}$ hour         

(4) $\frac{1}{4}\mathrm{hour}$

Unit of radioactivity is Rutherford. Its value is

(1) $3.7\times {10}^{10}$ disintegrations/sec

(2) $3.7\times {10}^6$ disintegrations/sec

(3) $1.0\times {10}^{10}$disintegrations/sec

(4) $1.0\times {10}^6$ disintegrations/sec

A radioactive substance has a half life of 60 minutes. After 3 hours, the fraction of atom that have decayed would be

(1) 12.5%               

(2) 87.5%

(3) 8.5%                 

(4) 25.1%

An element used for radioactive carbon dating for more than 5600 years is 

(1) C-14         

(2) U-234

(3) U-238       

(4) Po-94

N atoms of a radioactive element emit n alpha particles per second. The half life of the element is

(1) $\frac{\mathrm{n}}{\mathrm{N}}$ sec                   

(2) $\frac{\mathrm{N}}{\mathrm{n}}$ sec

(3) $\frac{0.693 \mathrm{N}}{\mathrm{n}}$ sec           

(4)  $\frac{0.693 \mathrm{n}}{\mathrm{N}}$ sec

The radioactive nucleus ${}_7{}^{13}\mathrm{N}$ decays to ${}_6{}^{13}\mathrm{C}$ through the emission of 

(1) Neutron             

(2) Proton

(3) Electron             

(4) Positron

The radioactive decay of uranium into thorium is expressed by the equation ${}_{92}{}^{238}\mathrm{U}\to {}_{90}{}^{234}\mathrm{Th}+\mathrm{X}$ where 'X' is

(1) An electron               

(2) A proton

(3) A deuteron                 

(4) An alpha particle