Question

Half life of a first order reaction is 4 s and the initial
concentration of the reactants is 0.12 M. The concentration
of the reactant left after 16 s is [1999]
(a) 0.0075 M (b) 0.06 M
(c) 0.03 M (d) 0.015 M

Answer 1

Answer:

It requires counting to decide the correct answer

Answer 2

The concentration of the reactant which is left is 0.0075M

Half life of reaction = 4 seconds (Given)

Concentration of reactant = a0 = 0.12 M (Given)

Second time = 16s (Given)

The concentration of the left over reactant is calculated as - a = a0/2n

where, a is the amount of reactant left after n-half lives, a0 is the initial amount of the reactant and n is the number of half lives.

Thus,  

Total time / Half life  

= 16/4  

= 4

Substituting the values -  

a = 0.12/2`4

= 0.12/16

= 0.0075M