JEE Main levelSingle correct optionDifficulty 2 of 3ck034
A first order reaction has k=0.00693s−1 (ln2=0.693). The time required for 87.5% of the reactant to react is
A300s
B175s
C200s
D400s
Answer: A
t1/2=0.693/0.00693=100s. Reacting 87.5% leaves 12.5%=(1/2)3, i.e. three half-lives: 3×100=300s. The value 175 s comes from wrongly scaling time with percentage reacted.