Question

A first order reaction is 20% complete in 30 minutes. Calculate the time to complete 80% reaction.

Its urgent plz help...​

Answer 1

Answer:

$a = a _{0} {e}^{ - \lambda \: t} \\ {a \over \: a _{0}} = {e}^{ - \lambda \: t} \\ log{a \over \: a _{0} } = { - \lambda \: t} \\ when \: \: t = 30 \: then \: 80\% \: left\: \\ log \frac{100}{80} = - \lambda(30) \\ when \: t = t \: let \: 20\% \: left \\ \\ log \frac{100}{20} = - \lambda(t) \\ divide \: both \: equations \\ t = 30 \times ( \frac{ log(5) }{ log( \frac{5}{4} ) } ) \approx 30 \times \frac{0.7}{0.7 - 0.6} = 210 \: minutes$