Question

A first order reaction is one-fifth completed in $40$ minutes. The time required for its $100$% completion is:

• A. $100$ minutes
• B. $200$ minutes
• C. $350$ minutes
• D. $\infty$ minutes

Answer 1

Answer: (D)

$\infty$ minutes

$k=\frac{2.303}{t}.log\frac{a}{a-x}$

$t=\frac{2.303}{k}.log\frac{a}{a-x}$

$t=\frac{2.303}{k}.log\frac{100}{0}$

$t=\frac{2.303}{k}.log\infty$

where 2.303/k is rate constant.

$t=\infty$

First order rate laws are never fully complete so $t=\infty .$