Answer
a) $1.0\times10^{4} s$
b) $3.4\times10^{4} s$
Work Step by Step
a) $t_{1/2}=\frac{0.693}{k}=\frac{0.693}{6.7\times10^{-5}s^{-1}}=1.0\times10^{4} s$
b) $k= \frac{2.303}{t}\log \frac{[R]_{0}}{[R]}$
Given that $[R]= \frac{[R]_{0}}{10}$
Therefore, $t= \frac{2.303}{k}\log\frac{[R]_{0}}{[R]_{0}/10}$
$=\frac{2.303}{k}\log 10= \frac{2.303}{k}$
$=\frac{2.303}{6.7\times10^{-5}s^{-1}}=3.4\times10^{4}s$
