Explanation

(a)
For first order reaction,
$t = \frac{2.303}{k}\log\frac{\lbrack A\rbrack_{0}}{\lbrack A\rbrack}$
$\lbrack A\rbrack_{0} = 5g,\lbrack A\rbrack = 3g\ k = 1.15 \times 10^{- 3}s^{- 1}$
$t = \frac{2.303}{1.15 \times 10^{- 3}}\log\frac{5}{3}$
$= \frac{2.303}{1.15 \times 10^{- 3}}\lbrack log5 - log3\rbrack$
$= \frac{2.303}{1.15 \times 10^{- 3}} \times 0.2218$
= 444s