Caculalate the mass of `.^(140)LA` in a sample whose activity is `3.7xx10^(10)Bq` (`1` Becquerel, `Bq= 1` disntegration per second) given that is `t_(1//2)` is 40 hour.
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Rate of decay `= lambda`
`N_(0) = (lambda xx w xx Av. No .)/(140)`
[Let `w` is weight of `La^(140)`, then `N_(0) = (w xx Av. No .)/(140)`]
`3.7xx10^(10) = (0.693)/(40xx60xx60) xx (w xx 6.02xx10^(23))/(140)`
`w = 1.79xx10^(-6) g`