Question

The half-life of 235U, an alpha emitter, is 7.1×108 yr. |
Calculate the number of alpha particles emitted by 3.4 mg of this nuclide in 3 minutes. Express your answer using two significant figures. |

Answer #1

**answer: 1.3*10^19**

**first order kinetics,**

**k = 0.693/t1/2**

** = 0.693/(7.1*10^8)**

** = 9.76*10^-10 y-1
k = (1/t)ln(a/x)**

**a = initial amount = 3.4 mg**

**x = amount of isomer after t, time = x mg**

**t= time = 3 min**

**(9.76*10^-10*365*24*60) = (1/3)ln(3.4/x)**

**x = 3.3948 mg**

**no of mol of 235-U disintegrated = (3.4-3.3948)/235 =
2.213*10^-5 mol**

**no of mol of alpha particel released = 2.213*10^-5
mol**

no of alpha particel released =
2.213*10^-5*6.023*10^23

**
= 1.3*10^19**

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