1.

10 g atoms of an alpha-active radioactive isotope aredisintegrating in a sealed container. In one hour, helium gas collected at STP is 11.2 "cm"^3. The half-life of the radioactive isotope is

Answer»

138.6 hr
1386 hr
13860 hr
138600 hr

Solution :Gram atoms of HELIUM gas formed in 1 hour`=11.2/22400=5xx10^(-4)`
i.e., Gram atoms of radioactive isotope disintegrated in one hour =`5xx10^(-4)`
RATE of DISINTEGRATION = `lambdaN`
`5xx10^(-4) = lambda xx 10` or `lambda=5xx10^(-5)hr^(-1)`
`t_(1//2) = 0.693/lambda=0.693/(5xx10^(-5))` hr = 13860 hr


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