Question

In a first order reaction, 10% of the reaction is completed in 1 hour. The percentage of the reactant remaining after 2 hours is:

• A. 80
• B. 60
• C. 50
• D. 81

Answer 1

Answer: (D)

81

In 1 hour, 10% of the reaction is completed. Thus, out of 100 moles, 90 moles remain. In the next 1 hour, another 10% of the reaction is completed.

Thus, out of 90 moles, 81 moles remained. Therefore, the percentage of reactant remaining after 2 hours is 81%.

$100\underset{1hr}{\overset{10\%}{\to }}90\underset{1hr}{\overset{10\%}{\to }}81$