PYQ LensCBSE Class 12 Chemistry PYQs
Chemistry papers mix reactions, numericals and reasoning. Practise real CBSE questions across physical, organic and inorganic chemistry, and learn how each step earns its marks.
- 20Papers
- 5Years
- 670Questions
Chapters with the most questions
| Chapter | Questions |
|---|---|
| Chemical Kinetics | 77 |
| Biomolecules | 74 |
| Electrochemistry | 73 |
| Solutions | 67 |
| Amines | 66 |
| Coordination Compounds | 65 |
| The d- and f-Block Elements | 64 |
| Haloalkanes and Haloarenes | 63 |
Counted across the 20 papers listed on this page.
Try a sample question
From the CBSE Class 12 Chemistry Sample Paper 2027.
If half-life of a first order reaction is 16 minutes, how much time does it take to complete 75% of the reaction?
- 16 min
- 32 min
- 48 min
- 64 min
Answer
B. 32 min
Explanation
A half-life is the time for the reactant amount to fall to half its current amount. For a first-order reaction at fixed conditions, each half-life has the same duration, here 16 minutes.
Start with a convenient 100 units. After 16 minutes, 50 remain and 50 have reacted. During the next 16 minutes, HALF OF THOSE 50 react: 25 react and 25 remain. Total reacted is then 50 + 25 = 75 units, or 75% of the initial amount.
75% complete means 25% remains. The remaining fraction is 25/100 = 1/4 = (1/2)², so two half-lives have elapsed. Time = 2 × 16 = 32 minutes. Final answer: B.
A gives only 50% completion. C is three half-lives, leaving 12.5% and completing 87.5%. D is four half-lives, leaving 6.25% and completing 93.75%. Each interval removes half of what remains, not another fixed 50% of the starting amount.