Answer
See the answer below.
Work Step by Step
a) For first-order processes:
$t_{1/2}=0.693/k\rightarrow k=0.0277\ min^{-1}$
$\ln(C/C_0)=-kt$, at constant volume, we can reduce the concentration ratio to a mass ratio:
$\ln(m/8.00\ g)=-0.0277\ min^{-1}\times t$
$m(125\ min)=0.25\ g$
$m(145\ min)=0.14\ g$
b) $\ln(2.25\ ng/7.60\ ng)=-0.0277\ min^{-1}t$
$t=43.9\ min$
c) $\ln(C/C_0)=-kt$
$\ln f=-0.0277\ min^{-1}\times 150\ min$
$f=0.016=1.6\%$
