Question

80% of a first order reaction was completed in 70 min. How much will it take for 90% completion of the reaction?

• A. 114 min
• B. 140 min
• C. 100 min
• D. 200 min

Answer 1

The correct option is C 100 min

$t_{80\%}=log\frac{100}{100-80}=log\frac{10}{2}$
$=log5=0.69\approx 0.7$
$t_{90\%}=log\frac{100}{100-90}=log10=1$
$\frac{t_{80\%}}{t_{90\%}}=\frac{0.7}{1}\Rightarrow \frac{70}{t_{90\%}}=\frac{0.7}{1}$
$\therefore t_{90\%}=\frac{70}{0.7}=\frac{700}{7}=100min$