Answer
a) $2.0×10^2d$
b) $2.7\times10^{-14}g$
Work Step by Step
$\frac{\Delta N}{\Delta t}=(\frac{\Delta N}{\Delta t})_oe^{-\lambda t}$
$\lambda=0.013075d^{-1}$
$t=\frac{-\ln(\frac{\Delta N/\Delta t}{(\Delta N/\Delta t)_o})}{\lambda}=2.0×10^2d$
$N=\frac{\Delta N/\Delta t}{\lambda}=2.3128\times10^9$atoms
$m=\frac{7g}{mol}\times\frac{1mol}{6.02\times10^{23}atoms}\times2.3128\times10^9$atoms$=2.7\times10^{-14}g$
