Chemistry · Some Basic Concepts Of Chemistry · NEET
Use moles = Molarity × Volume in litres. The trap is the volume must be in litres, not mL. So first divide the mL by 1000. Example: 50 mL of 0.5 M HCl means moles = 0.5 × (50/1000) = 0.5 × 0.05 = 0.025 mol. Always convert mL to litres before multiplying.
A balanced equation talks in MOLES, not in mL or grams. The coefficients (like 2HCl) mean 2 moles, not 2 mL. So every solution amount must first become moles. The full path for NEET is: (M × V) gives moles of what you know, then the mole ratio from the balanced equation gives moles of what you want, then multiply by molar mass to get grams if the question asks for mass.
Look at the coefficients: 1 CaCO3 reacts with 2 HCl. So moles of CaCO3 = moles of HCl ÷ 2. If you have 0.025 mol HCl, then CaCO3 = 0.025 ÷ 2 = 0.0125 mol. Never add the coefficient as a multiplier by mistake here; because HCl has the bigger coefficient (2), you divide.
Multiply moles by the molar mass. Mass = moles × molar mass. For 0.0125 mol of CaCO3 (molar mass 100 g/mol), mass = 0.0125 × 100 = 1.25 g. This is the last step, and only do it if the question asks for grams or mg.
Purity is the very last adjustment. First find the mass of PURE substance needed (1.25 g here). Then a 95% pure sample means only 95% of it is the real substance, so you need MORE total sample: mass = pure mass × (100 / percent purity) = 1.25 × (100/95) = 1.32 g. Divide by the purity fraction; the impure mass is always larger than the pure mass.
Convert both reactants to moles first. Then divide each mole amount by its coefficient in the balanced equation. The one with the smaller value runs out first and is the limiting reagent. For NaOH + HCl (1:1), just compare the moles directly; the smaller one limits the reaction and controls how much product forms.
What mass of 95% pure CaCO3 is required to neutralise 50 mL of 0.5 M HCl according to the reaction: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + CO2(g) + 2H2O(l)?
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.
Moles = Molarity (M) × Volume of solution in litres. Rearranged, Molarity = moles ÷ volume(L). This is the key bridge for every solution stoichiometry problem in NEET.
Molarity is defined as moles per litre of solution. So the formula moles = M × V only works when V is in litres. If your volume is in mL, divide by 1000 first (for example, 50 mL = 0.050 L).
Molarity itself depends on temperature because the volume of a solution expands or contracts with heat. But for a standard NEET stoichiometry problem at one temperature, you use the molarity value given directly; you do not adjust it.
NEET gives you a volume and molarity of one solution, a balanced equation, and asks for the mass of another reactant or the unreacted amount. Real examples are the 2022 CaCO3+HCl question and the 2024 NaOH+HCl unreacted-mass question.
The next natural step is dilution, where you use M1V1 = M2V2 to find the new concentration or volume after adding water. It uses the same idea that moles stay constant.
