A catalyst bed produces $8.0 mol$ of product per hour when fresh. After a poison blocks $70%$ of its active sites, the rate is proportional to the fraction of free sites. The time needed to make $5.04 mol$ of product with the poisoned catalyst is

Answer: A

Free sites

= 30%

, so rate

= 0.30 \times 8.0 = 2.4 mol h^{- 1}

. Time

= 5.04/2.4 = 2.1 h = 126 min

. The value 54 min wrongly uses

70%

of sites and 37.8 min ignores poisoning.