Question

A first order reaction is $40$% complete after $8$ $min$. How long will it take before it is $90$% complete? What is the value of the rate constant?

Answer 1

$k=\frac{2.303}{t}\log \frac{[A{]}_0}{\left[A\right]}$
$k=\frac{2.303}{8}\log \frac{\left[10\right]}{\left[6\right]}$
$k=\frac{2.303\times 0.22}{8}$

Now, $t=\frac{2.303\times 8}{2.303\times 0.22}\log \frac{\left[10\right]}{\left[1\right]}$
$t=36.36$ minutes