Question

The thermal decomposition of a compound is of first order. If 50% of a sample of the compound is decomposed in 120 min how long will it take for 90% of the compound to decompose ?

• A. 399 min
• B. 410 min
• C. 250 min
• D. 120 min

Answer 1

The correct option is A 399 min

$k=\frac{0.6932}{120}$ (i)
$k=\frac{2.303}{t}log\frac{a}{0.10a}$
$=\frac{2.303}{t}log10$ (ii)
Equating (i) and (ii).
$\frac{0.6932}{120}=\frac{2.303}{t}$
t = 399 min