A first order reaction has a specific reaction rate of 10-2s-1. How much time will it take for 20 g of the reactant to reduce to 5 g?
(a) 238.6 s (b) 138.6 s (c) 346.5 s (d) 693.0 s
(b) For a first order reaction,
Rate constant (k) = 2.303/t . log(a/a-x)
where, a = initial concentration
a-x = concentration after time 't'
t= time in 'sec'
Given, a= 20 g, a-x = 5g, k=10-2
... t =2.303/10-2 . log(20/5) = 138.6 s
Half-life for the first order reaction,
t1/2/2 = 0.693/k = 0.693/10-2 = 69.3s
Two half-lives are required for the reduction of 20 g of reactant into 5g