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Question

# At 300 K, a gaseous reaction A→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 reaction is 180 mm of Hg. The partial pressure of A (in mm of Hg) is :

A
100
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B
90
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C
180
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D
80
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Solution

## The correct option is B 80Let Po be the initial pressure of A. The total pressure after the completion of the reaction is 180 mm of Hg. It is equal to 2Po (because for every mole of A, one mole of B and one mole of C will be produced). Hence, total pressure at the end of the reaction will be twice the initial pressure of A.2PA=180 mm Hg.PA=90 mm Hg.The total pressure at the end of 20 minutes was 100 mm of Hg. The total pressure is Po−x+x+x=Po+x where x is change in the partial of A to form the products at the end of 20 minutes .Po+x=100 mm HgButPo=90 mm HgHence, 90+x=100x=10 mm HgThe partial pressure of A (in mm of Hg) is Po−x=90−10=80 mmHg.

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