Chemistry · Some Basic Concepts Of Chemistry · NEET
Follow 3 steps. Step 1: Balance the equation. Step 2: Convert the given mass of each reactant to moles (moles = mass / molar mass). Step 3: Divide each reactant's moles by its coefficient (the big number in front of it in the balanced equation). The reactant with the SMALLEST value is the limiting reagent. Do not just compare grams or raw moles — you must divide by the coefficient first, because reactants are used in a fixed ratio.
Because reactants combine in a fixed ratio, not one-to-one. In N2 + 3H2 -> 2NH3, one N2 needs three H2. If you had 1 mol N2 and 2 mol H2, H2 has more moles but it still runs out first, because 1 mol N2 needs 3 mol H2 and you only have 2. That is why you divide moles by the coefficient before comparing. The smallest result, not the smallest mole count, is limiting.
The limiting reagent decides the maximum amount of product. Once it is used up, the reaction stops, even if other reactants are still left. So you ALWAYS calculate the product amount from the limiting reagent, never from the excess one. Many NEET stoichiometry questions give you two reactant masses on purpose — if you use the wrong reactant, you get the wrong product mass and lose the mark.
The limiting reagent is fully consumed and stops the reaction; it controls the product. The excess reagent is the one left over after the reaction ends; some of it stays unreacted. In a NEET question, first find which is limiting, use it to find the product, then find how much of the excess reagent was used and subtract to get the leftover amount.
After finding the limiting reagent, use the balanced ratio to calculate how many moles of the EXCESS reagent actually reacted. Then subtract that from the moles you started with. The difference is the unreacted (leftover) amount. Convert back to grams if the question asks in grams. This exact idea was tested in NEET 2024 (NaOH left unreacted).
Always in moles, never in grams. Grams do not tell you how many particles are present, and the balanced equation is written in moles (particles). So convert every mass to moles first, then divide by the coefficient. Comparing grams directly is a common trap and gives the wrong limiting reagent.
1 gram of NaOH is treated with 25 mL of 0.75 M HCl solution. The mass of NaOH left unreacted is:
The number of moles of hydrogen molecules required to produce 20 moles of ammonia through the Haber process is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Mass alone does not decide it. You must convert to moles and divide by the coefficient. A reactant with a larger mass can still be limiting if its molar mass is high or its coefficient is large.
Yes. If the reactants are mixed in exactly the ratio shown by the balanced equation, both run out at the same time. Then neither is in excess and there is no leftover reactant. This is called a stoichiometric mixture.
Always from the limiting reagent. It is used up first and stops the reaction, so it fixes the maximum product amount. Using the excess reagent gives a wrong, too-large answer.
In purity problems, only part of a sample is the real reactant. In limiting reagent problems, you compare two or more pure reactants to see which finishes first. A hard NEET question can combine both, so master each separately.
If the question gives you amounts (mass, moles, or volume x concentration) for TWO or more reactants, it is almost always a limiting reagent question. When only one reactant amount is given, it is simple stoichiometry.
