Chemistry · Some Basic Concepts Of Chemistry · NEET
It is the reactant that gets used up first. Once it is gone, the reaction stops, even if a lot of the other reactant is still left over. Because it runs out first, it controls (limits) how much product you can make. NCERT says: the reactant present in the least amount gets consumed and limits the amount of product formed.
Because reactants do NOT react 1:1 in most equations. A balanced equation may need 3 moles of one reactant for every 1 mole of another. So you must FIRST divide each reactant's moles by its own coefficient. Only then does the smallest value tell you the limiting reagent. Picking the smaller mole count without dividing is the most common NEET mistake.
Step 1: Balance the equation. Step 2: Convert every given amount (grams, mL, molarity) into moles. Step 3: Divide each reactant's moles by its coefficient in the balanced equation. Step 4: The reactant with the SMALLEST result is the limiting reagent; the other is in excess. Step 5: Use the limiting reagent's moles for all product calculations.
For a solid mass, moles = mass in grams / molar mass. For a solution, moles = molarity (mol/L) x volume in litres. Example: 1 g NaOH = 1/40 = 0.025 mol. And 25 mL of 0.75 M HCl = 0.025 L x 0.75 = 0.01875 mol. Always convert to moles before comparing.
You use ONLY the moles of the limiting reagent to calculate product formed or excess left. The excess reagent's extra amount just sits unreacted. To find how much excess is left over, see the next concept: finding mass of unreacted (excess) reagent.
The limiting reagent runs out first and decides the product amount. The excess reagent is the one left over after the reaction stops - part of it never reacts. In the NaOH + HCl NEET question, HCl is limiting and NaOH is in excess, so some NaOH stays unreacted.
1 gram of NaOH is treated with 25 mL of 0.75 M HCl solution. The mass of NaOH left unreacted is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. It is the one with the smallest value of (moles / coefficient). If the equation is 1:1 (equal coefficients), then yes, fewer moles means limiting. But when coefficients differ, you must divide first.
Yes. Product formed is calculated only from the moles of the limiting reagent, using the mole ratio from the balanced equation. The excess reagent's leftover amount does not add any product.
NEET usually gives you grams or a molarity and volume, asks for product formed or mass of reactant left. You must find the limiting reagent first. The 2024 NaOH + HCl question is a direct example.
Only if they are present in the exact stoichiometric ratio - then neither is left over and there is no excess. This is a special balanced case, not the usual limiting-reagent problem.
