Question

A first order reaction complete its 10% in 20 minutes then time required to complete its 19% is.

• A. 30 minutes
• B. 40 minutes
• C. 50 minutes
• D. 38 minutes (5) 45 minutes

Answer 1

Answer: (B)

40 minutes

Answer 2

In first phase, $K = \frac{2.303}{20}\log\frac{90}{100}$ .....(i)

In second phase $K = \frac{2.303}{t}\log\frac{81}{100}$ .....(ii)

From eq. (i) $\frac{2.303}{20}\log\frac{90}{100} = \frac{2.303}{t}\log\frac{81}{100}$

$t = \frac{20(\log 81 - \log 100)}{(\log 90 - \log 100)}$

$= \frac{20(1.908 - 2)}{(1.954 - 2)} = \frac{20 \times ( - 0.092)}{( - 0.046)}$= 40 minutes