Track amounts, ratios and chemical change
Keep moles, concentration, reaction ratios and equilibrium effects distinct.
The idea
Chemical quantities describe different things. Moles count an amount; molarity divides that amount by the solution volume. A balanced equation supplies mole ratios, so comparing reactant masses or mole counts alone does not locate the limiting reactant. A change in concentration can also change a reaction quotient without changing the equilibrium constant at fixed temperature. Write the relevant balance before calculating or choosing a direction.
A method to try
- Balance the reaction, or write the concentration or charge relationship that the problem requires.
- Convert mass to moles and volume to litres where appropriate. Keep units beside the numbers.
- For competing reactants, compare each available amount divided by its reaction coefficient.
- For dilution, preserve solute amount. For Hess's law, cancel species. For equilibrium, compare the reaction quotient with the unchanged constant at fixed temperature.
Follow the reasoning.
For 2H₂ + O₂ → 2H₂O, a mixture contains 0.50 mol H₂ and 0.20 mol O₂. Assume the limiting reactant reacts fully. Find the water formed and the reactant left over.
- Divide each amount by its coefficient: hydrogen gives 0.50/2 = 0.25 and oxygen gives 0.20/1 = 0.20. Oxygen permits the smaller reaction extent and is limiting.
- The reaction produces 2 × 0.20 = 0.40 mol H₂O and consumes 0.40 mol H₂. Hydrogen remaining is 0.50 − 0.40 = 0.10 mol.
- Check the atomic balance using the reacted amounts: 0.40 mol H₂ supplies 0.80 mol H atoms, and 0.20 mol O₂ supplies 0.40 mol O atoms. The 0.40 mol H₂O product contains exactly these amounts.
Think it through first.
A 0.30 mol/L solution is diluted from 20 mL to a final volume of 60 mL without losing solute. What is the new concentration?
Show the explanation
0.10 mol/L.
The volume triples while the solute amount stays fixed, so concentration is divided by three: 0.30 × 20/60 = 0.10 mol/L.
Make it your own
Label every intermediate quantity as an amount, concentration, ratio or energy. If your units change unexpectedly, stop and repair the setup before continuing.
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 4 references and checked dates
- NTA: NEET (UG) 2026 Information Bulletin (opens in a new tab)Checked 2026-09-29
Original February 2026 bulletin. Later re-examination notices must also be read; this is not a 2027 announcement.
- NTA/NMC: NEET UG 2026 syllabus notice and annexure (opens in a new tab)Checked 2026-09-29
NTA notice dated 8 January 2026 encloses the NMC syllabus notified 22 December 2025.
- NTA: official NEET UG portal and notices (opens in a new tab)Checked 2026-09-29
Check current notices for a new application cycle; historical notices are not open application windows.
- NTA: 21 June 2026 re-examination notice, 15 May 2026 (opens in a new tab)Checked 2026-09-29
Page 1 gives the 14:00–17:15 IST window including formalities, pen-and-paper mode and no re-examination fee.