At 300 K a gaseous reaction `A rarr B +C ` was found to follow first order kinetics . Starting with pure A the total pressure at the end of 20 minutes was 100 mm of Hg . The total pressure after the completion of the reactionn is 180 mm of Hg . The partial pressure of A ( in mm of Hg ) is :

Options

A. 100
B. 90
C. 180
D. 80

Answer

B

Explanation

(B) `A(g) rarr B(g)+C(s)`
At t=0 min `" " P " " 0" "0`
At t=20 mm `" " P-p" "p" "p`
After Completion `" "0 " " p" "P`
Total pressure after completion of reaction
= P+P
= 180 mm Hg
or 2P = 180 mm Hg
or P = 90 mm Hg
`:. ` Partial pressure of A is 90 mm Hg .