Daily answer writing · Mon, 19 Oct, 2026
First-order reaction: show t₉₉.₉% is about ten half-lives, and a numerical
Derive the integrated rate equation for a first-order reaction and show that the time required for 99.9% completion is nearly ten times its half-life. A first-order reaction is 20% complete in 15 minutes. How long will it take to reach 75% completion?
Syllabus: Common to CSE Paper I, IFoS Paper I and Geo-Scientist Mains (physical): chemical kinetics, integrated rate laws.
Model answer
This model answer was free for 7 days after it appeared. Older answers are in the archive, which is part of the VKC plan. Today's question and the last week's answers are always free.
Part of VKC Basic
Read this model answer and the whole archive with VKC Basic
VKC Basic (₹999 for 6 months) opens every solved answer, the notes, the handbook and the daily answer archive. Every Mains test series includes it.