Question:

A first order reaction has a specific reaction rate of 10⁻¹ sec⁻¹. How much time will it take for 20 g of the reactant to reduce to 5 g?

238.6 sec

346.5 sec

693.0 sec

138.6 sec

The correct option is C

138.6 sec
t = 2.303/k log(a/a-x)
t = 2.303/10⁻¹/s log(20g/5g)
t = 138.6 sec

Hence, the time required will be 138.6 sec.