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Solution


Q.

Reaction A(g) → 2B(g) + C(g) is a first order reaction. It was started with pure A.

t / min Pressure of system at time t / mm Hg
10 160
∞ 240

 

Which of the following options is incorrect                          [2025]
1 Initial pressure of A is 80 mm Hg.  
2 The reaction never goes to completion.  
3 Rate constant of the reaction is 1.693 min-1.  
4 Partial pressure of A after 10 minutes is 40 mm Hg.  

Ans.

(3)

A(g)⟶2B(g)+C(g)t=0 (pressure)p°00t=10 minp°-p2ppTotal=p°+2p=160-(I)t=∞0 2p° p° Total=3p°=240-(II)


From equations (I) and (II): 

p°=80 mmHg

p=40 mmHg

Thus, initial pressure of A (p°)=80 mmHg. 

Pressure of A after 10 min

p°-p=(80-40)mm Hg=40 mmHg

First-order reactions never go to completion.

Rate constant:

k=2.303tlogp°p°-p

    =2.30310log8080-40

   =2.30310log2   

=2.30310×0.3=0.06909 min-1