Track amounts and repeated changes
Apply mole ratios and half-lives.
The idea
A balanced reaction fixes mole ratios, not equal reactant amounts. For a first-order process, each half-life multiplies the remaining amount by one half.
A method to try
- Find which reactant supports the smaller product amount.
- Count half-lives before calculating the remaining fraction.
Follow the reasoning.
A first-order process has a half-life of 15 minutes. Starting with 80 units, how much remains after 45 minutes?
- The elapsed time contains three half-lives. Track 80 → 40 → 20 → 10 units.
- Equivalently, use 80 × (1/2)³ = 10. It is a repeated fractional decrease, not subtraction of the same number of units each time.
- The half-life statement is sufficient here because the process is first-order. Do not assume a constant half-life for every possible kinetic model. Check the model named in the question.
Think it through first.
For 2A + B → product, which reactant limits a mixture containing 4 mol A and 3 mol B?
Show the explanation
A.
Four moles of A require only two moles of B; one mole of B would remain after A is used.
Make it your own
After reading, explain the method in your own words and identify the condition that makes it valid. Then try the linked starter practice without referring back to the worked example. Review the reasoning behind an error before repeating the same item; remembering an answer is different from being able to solve a new problem.
This lesson is one part of a short starter pathway. It does not cover the full exam syllabus or establish a predicted score.
Official references
These provider and publisher links explain the exam or underlying concepts. Our lessons and questions are original, independent practice.
View 8 references and checked dates
- NTA: 2026 information bulletin (opens in a new tab)Checked 2026-09-20
Observed PDF linked as IB 2026 from the official bulletin page; format on printed pages 11–12.
- NTA: 2026 syllabus (opens in a new tab)Checked 2026-09-20
Observed PDF linked from the official JEE Main site.
- NTA: JEE Main portal (opens in a new tab)Checked 2026-09-20
- OpenStax Calculus Volume 1 — 5.5 Substitution (opens in a new tab)Checked 2026-09-20
Concept and scope reference. Our questions and explanations are original; no provider endorsement.
- OpenStax University Physics Volume 3 — 6.2 Photoelectric Effect (opens in a new tab)Checked 2026-09-20
Concept and scope reference. Our questions and explanations are original; no provider endorsement.
- OpenStax Chemistry 2e — 11.4 Colligative Properties (opens in a new tab)Checked 2026-09-20
Concept and scope reference. Our questions and explanations are original; no provider endorsement.
- OpenStax Chemistry 2e — 13.4 Equilibrium Calculations (opens in a new tab)Checked 2026-09-20
Concept and scope reference. Our questions and explanations are original; no provider endorsement.
- OpenStax Chemistry 2e — 7.6 Molecular Structure and Polarity (opens in a new tab)Checked 2026-09-20
Concept and scope reference. Our questions and explanations are original; no provider endorsement.