NEET Chemistry Chemical Kinetics Questions Solved

NEET - 2017

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

Alternatively,

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

    20g t1/210gt1/2

 

Difficulty Level:

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